The Hackage package search as of now doesn't just give a list of all matches as it used to do, but allows sorting them in various ways. Notably, it is now possible to sort by rating, i.e. by how satisfied users were with the package (if people will actually start rating packages... this has been possible for a while but almost no votes are assigned yet), and by some “DL” criterium. I first assumed this somehow counted the reverse dependencies, i.e. described how many other packages depend on this one, but that can't seem to be the case since base has only a value of 1692 whereas e.g. base64-bytestring has a whopping 7384.
So if that's not it, what does this number tell me?
It's downloads.
Clicking through to base for example shows:
Downloads 21206 total (1692 in the last 30 days)
So it appears it's the last 30 days download count.