How do I flatten my uneven concrete floor?
You are looking for leveling compound for concrete.  That link is to one of many of Home Depot's choices for a fast setting mix which can be up to one inch thick.  The reviews say the instructions should be carefully followed; many rave about the results.  I imagine if you mucked up the application, it could be difficult to correct.  (I have only done leveling compound on top of a wood subfloor:  that was pretty easy to get right.)