In short, the Kaaba is the large, black cuboid shaped building at the centre of the Holy Mosque (Masjid Al Haram) in Makkah, Saudi Arabia. It has a long history but in short, it is mentioned in the Quran: “The first House (of worship) appointed for men was that at Bakka: Full of blessing and of guidance for all kinds of beings:” (Surah Aal Imran 3:96)
In pre Islamic times, it was filled with idols so Muslims did not face towards it during prayer. After Makkah came under Muslim rule and everyone there became Muslim, all the idols were removed and Muslims today around the world face towards it during prayer so it is the “Qibla”. It is therefore also a form of unity as Muslims all over the world -East and West, face towards it when they pray.
You can find out more about the Kaaba, its history and significance from this free e-book, Kaaba Unknown, by Sheikh Waleed Basyouni:
Muslims face towards this but they do not worship it. In fact, every year groups of people stand on top of the Kaaba and change the black cloth covering it and that is not something that people can do with idols. Also, people can enter it and pray inside -although this is done very rarely with certain visitors only. You can get an idea of what it looks like from the inside from here: