It means that it will work better on a 64-bit OS than on a 32-bit OS. But it’s not a 64-bit application.
For example, a 64-bit OS can see more than 4GB of RAM, so several processes running simultaneously can use a total of far, far more than 4GB of RAM. That means that After Effects can actually get a bigger hunk of a 4GB block of RAM and not have to share it with as much other software. It also means that multiple processes of After Effects can each get a 4GB block of RAM.
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Todd Kopriva, Adobe Systems Incorporated
putting the ‘T’ back in ‘RTFM’ : After Effects Help on the Web
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