Anıtkabir is the mausoleum of Mustafa Kemal Ataturk, the founder of the Turkish Republic. The place is very significant and the Independence Day holiday is held here first of all.
If possible, you can see the solemn changing of the guard post by the soldiers. The post is changed every 1-2 hours (as it says on the Internet).
The building itself is without frills and pomp. This gives a very clear understanding of Ataturk's policy.
The place is iconic for Turks, with cars, Ataturk's yacht, and the artillery carriage that carried his coffin on display. You can take a walk, but I won't put visiting Ankara in the first place.