Spacing is when you keep a certain distance from your opponent and control it using specific attacks at specific ranges. For example when Ein is at mid range if I try to poke with 3K, or 1K thats an example of spacing. Easiest way to see what I'm talking about is to watch any vid of me playing Ein. What you see me doing is spacing.
Free Step Dodge
Tenryuga
Force_of_Nature
Ten's spacing & turtling is godlike. His nickname is Mr. Timeout IIRC.