"Awaiting Source" means that the host(s) of the file you're trying to download is(are) not online.
"Need Source" means that no host has been found for the file you're trying to download at the time you're online.
So, let's say you open LW and you have some files from an earlier search in your download window. If the host(s) of the file is online, LW will try to connect you with it. If LW doesn't get a positive respond (i.e. no hosts are online for that file), it will show you "need source(s). If you click "find source" LW will try to find online hosts for your file. If it can't find any, it will start a time counter (usually 300 sec.) after which it will go to awaiting source. It will check frequently whether the host(s) of the file has come online.
