It means Righteousness/Purity which is the same as in Hebrew. This is an
Alms Tax which is one of the Five Pillars of the Muhammadan creed and hence is mandatory.

       No other practical duty is so repeatedly mentioned both in the Quran and the Ahadith in emulation of the Jewish doctrine that almsgiving atones for sins. This deed is always associated with the exhortation to prayers. This concept too, is copied from the tradition of the Jews as stipulated in the Torah and in Daniel 4:24 and the book of Tobit 4:7-11; 16f .

       The Quran, faithfully reproduces these precepts as shown in the following verses
(circa second year of Hijra):

2:   43        And be steadfast in prayer; practice regular charity; and bow down your heads with those who bow down (in worship)….

     270        And whatever ye spend in charity or devotion be sure Allah knows it all.  But the wrong-doers have no helpers.

     271                If ye disclose (acts of) charity even so it is well but if ye conceal them and make them reach those (really) in need that is best for you: it will remove from you some of your (stains of) evil.  And Allah is well acquainted with what ye do         

9:   60        Alms are for the poor and the needy and those employed to administer the (funds); for those whose hearts have been (recently) reconciled (to truth); for those in bondage and in debt; in the cause of Allah; and for the wayfarer: (thus is it) ordained by Allah and Allah is full of knowledge and wisdom….

     103        Of their goods take alms that so thou mightest purify and sanctify them; and pray on their behalf.  Verily thy prayers are a source of security for them: and Allah is one who heareth and knoweth.

58: 13        Is it that ye are afraid of spending sums in charity before your private consultation (with him)? If then ye do not so and     Allah forgives you then (at least) establish regular prayer; practice regular charity; and obey Allah and His Apostle: and Allah is well-acquainted will all that ye do.

       Also in 2: 177, 215, 263, 264, 273, 274, 277, 280 ; 9:58; 22:35; 35:29; 58:12; 73:20; etc etc.

       The amount to give is stated in the Quran in 2:219-
       "as much as you can spare".

       After he had established his authority in Madina, Muhammad needed a common fund to care for the poor and orphans as well as to finance his war machine; for this necessity, a new 'revlation' was required to abrogate the earlier one.

       This fund was called the Khums/Fifth, which means that Muhammad - and Allah - were allocated 20% of all Booty & PLUNDER to finance Muhammad and his followers in their attempt to wage  war and subjugate all the inhabitants of Arabia to the will of Muhammad and his new CULT of 'ISLAM'.

       *** The reader should be made aware that since Allah did NOT claim his 10% share of the Booty & Plunder, all these 'rewards', all 20% were at the disposal and personal use of Muhammad ***

       The following verse fitted the requirement perfectly:

8: 41        And know that out of all the booty that ye may acquire (in war) a fifth share is assigned to Allah and to the apostle and to near relatives orphans the needy and the wayfarer if ye do believe in Allah and in the revelation We sent down to Our servant on the day of testing the day of the meeting of the two forces.  For Allah hath power over all things".

       It should be noted that this Quranic regulation is not concerned with individuals, but with the wealth acquired by the state. This is reminiscent of the story of Joseph - which Muhammad knew very well and enjoyed so much that he allocated a complete Surah 12 for it - who preceded Muhammad and his 'Islam' by at least 2200 years.

       It was the Muslim Joseph after all, who legislated concerning state property when he designated one fifth of all produce in the land of Egypt for its ruler, the Pharaoh
Gen. 47:24-26.

       The 20% figure is derived from the rabbinical ordinance which sets one fifth as the maximum for charity

Kethuboth 50a " He who will spend (his property in charity) must not spend more than one fifth part".
Similarly Jer.Peah 15a.

       As the 'Islamic' polity became an Empire, rules and regulations had to be made by her jurists to fix a precise system of payments and donations. The primary beneficiaries of these taxes were supposed to be the orphans, widows and poor.

       This concept of charity did not exist in pagan Arabia. It is copied entirely - as usual in Muhammad's Quran - from the Bible

Exodus 22:21 "You shall not afflict any widow, or orphaned child. 
                      22 If you afflict them in any wise, and they cry to me, I will surely hear their cry;
and Hebrew Scriptures.

       The Quran even copies the Rabbinical injunctions that such payments should be made discreetly, without drawing attention to the giver and that almsgiving atones for sin

Rabbi Judah in Baba Bathra 10 " great is almsgiving that it brings redemption near…"