Spread
A spread is a handicap applied to one side so that both sides of a market carry roughly equal chances.
In plain English
The favourite must win by more than the spread; the underdog must lose by less than it, or win. The purpose is to turn a lopsided matchup into a near coin flip at a stated price.
Example
A favourite at -6.5 must win by seven or more. Winning by six loses that bet, however comfortable the game looked.
Why it matters
Most sides markets in football and basketball are spreads, so the margin matters rather than just the result.