I have seen a lot of discussions about presale wallets, but have no idea what it means.
What do the terms "presale" and "presale wallet" mean in Ethereum?
The presale ran from 22nd of July 2014 to 2nd September 2014. This was before the general public launch of the network.
For the first couple of weeks, 2000 ETH cost 1 BTC. Thereafter, your 1 BTC gradually got you fewer ETH. See:
The "presale wallet" was the keystore file loaded with the ETH each participant purchased, which could then be downloaded.
See Launching the Ether Sale blog post for more information.