This is going to sound like a stupid question, but what is the source SDK base?

its basically the "base" files for HL2 so you can make your own mods based on the hl2 version of the source engine. basically unless you plan on making a mod. delete it. i installed it not knowing what it was. then once i found out, i deleted it

It has a nifty FPS test in it, which is about it unless you are creating a mod.

Thanks I haven't installed it because I want to know what things are before I waste my internet.

It has the source code for the engine so that you can make mods. If you're not going to be doing any programming, don't bother with it.