Shirk (شرك or širk) is the sin of practicing idolatry or polytheism, i.e. the deification or worship of anyone or anything besides Allah. Islamic scriptures say Allah will forgive every crime of Muslims but not shirk. Literally, this word means ascribing or establishing "partners" next to Allah. Muslims worship a black stone at Mecca. They kiss it during Hajj. So they themselves commit shirk. But what does the stone represent? Nothing other than some unproven Quranic and hadith legends.
In fact, Allah himself is a polytheist.