They are places - such as graves - regarded as holy because of their association with a divinity or a scared person or relic, typically marked by a building or other construction.
Muhammad repeatedly - as shown in the few references below - prohibited his followers from making his own grave, and any other for that matter, as a shrine to be visited and/or worshipped.
His followers, especially those of the Shia sect did and still do the opposite. The Wahabis of Saudi Arabia consider the acts of venerating 'Muslim' saints as sacrilegious since it is a species of Shirk, the cardinal sin of worshipping created beings along with Allah. They actually consider all non Wahabis to be unbelievers and deserving of Jihad and killing.
Sahih Al-Bukhari HadithHadith 1.419/426Narrated byAisha
Um Habiba and Um Salama mentioned about a church they had seen in Ethiopia in which there were pictures. They told the Prophet about it, on which he said, "If any religious man dies amongst those people they would build a place of worship at his grave and make these pictures in it. They will be the worst creature in the sight of Allah on the Day of Resurrection."
Sahih Al-Bukhari HadithHadith 2.472Narrated by Aisha
Allah's Apostle in his fatal illness said,
"Allah cursed the Jews and the Christians, for they built the places of worship at the graves of their prophets."
And if that had not been the case, then the Prophet's grave would have been made prominent before the people. So (the Prophet ) was afraid, or the people were afraid that his grave might be taken as a place for worship.
Sahih Al-Bukhari HadithHadith 5.725Narrated byUrwa bin Az Zubair
'Aisha said, "The Prophet said during his fatal illness,
'Allah cursed theJews for they took the graves of their prophets as places for worship.' " 'Aisha added, "Had it not been for that (statement of the Prophet) his grave would have been made conspicuous. But he was afraid that it might be taken as a place for worship."
Al-Muwatta Hadith Hadith 9.88
Yahya related to me from Malik from Zayd ibn Aslam from Ata ibn Yasar that the Messenger of Allah, may Allah bless him and grant him peace, said, "O Allah! Do not make my grave an idol that is worshipped. The anger on those who took the graves of their Prophets as places of prostration was terrible."