Glossary
Spacing
In one line: The right distance between teammates: close enough to trade, far enough not to die together.
What it means
Spacing is how you spread out while moving or holding an area. Too close, and one ability (a molly, an ultimate) or one well-placed enemy kills you both. Too far, and whoever dies is not avenged because the teammate cannot get there in time for the trade.
The right distance lets the second player see and hit whoever killed the first within a second or two. You learn it by playing and by watching where your teammates die.
An example
Two of you walk into a corridor. You are two steps behind your teammate and slightly to the side: when they get shot from a corner, you see the shooter at once and kill them.