Explain the concept of buffering

hello i am currently trying to do all these combos i see on this youtube vid
[media=youtube]Pn9iWx_XL9g[/media]
the one i am trying to do are seconds: :47 , 1:47, 2:03, 2:39 3:24
i see that in order to do some of these combos you have to understand the concept of buffering can anyone explain this seemingly simple but advanced concept of comboing

oh yeah i be trying to get on ggpo but i forgot my name and password so i am screwed when it come to logging on