Credit is a monetary system for obtaining pokemon. Normally, one credit is worth one pokemon, but it's at the thread starter's discretion as to how much they're worth. For instance, redistributables are always either one credit or half of one credit whereas OT pokes are either 1., 1.5, or 2 credits.
You don't need a trade thread to trade on Smogon, but some people only trade with those with a thread. Read/skim the thread owner's rules before conducting a trade to make sure ^_^
