Source SDK Base is an appid setup for mods that need to use the latest shipping version of the Source Engine, without requiring HL2 or HL2DM. It contains the same video stress test that shipped in Lost Coast. MOD's based on the Source SDK Base will update periodically in lock-step with engine updates.
In simple terms, this means that any mod built using 'Source SDK Base' use the latest Source Engine updates when they're released.