Aenima is Tool's major breakthrough work. Compared to its predecessors, Aenima is a completely different beast. It is a lot more complex and harder to digest. It may take months or even years to fully absorb it, and you are well advised to start with Undertow if you have never heard any Tool before. Once you've internalised their earlier work, you can come to Aenima and spend quite some time with it. Otherwise, like some people tend to do, you may just write them off as a pretentious act who are terribly overrated by their fan base, which would be a big loss for you.