In a nutshell it means not feeling any sexual attraction towards people but in the end is much more complex than that. Like everything else it's a spectrum and there isn't a 0 or 100. Also unlike many make it out to be, it is not abstinence nor does not mean that you don't feel ANY attraction at all.
As for the several types you have many, some are 100% asexual while others can be gray-ace (feel some attraction but in a low level and very rarely but it's there), demi-sexual (where you only start feeling sexual attraction when get an emotional connection with someone and not before), etc...
Also you have your sex positive aces (will have sex for many reasons feels good, want to please their partner, are demi, etc) neutral (don't care either way, they feel ok with both) and repulsive (hate any kind of intimacy of the sorts and are the ones that everyone seems to think of when they think of aces).
I feel that the hardest thing to distinguish is aesthetic and sensual attraction from sexual but the image bellow helps a bit but honestly you don't need to label it if don't feel like it but just know that there are many people who also feel the same around.
