A point spread means that each outcome has either a plus or minus handicap applied.
This plus/minus handicap is often referred to as the "line". For example, if a team is +3.5, then at the end of the game 3.5 points will be applied to their total. The aim of the spread is for the sportsbook to level the playing field and have each team have a roughly equal chance.
The winner of the game is the team that "covers the spread". If you bet a minus handicap, the team has to get within that number of points of their opponent.
The spread is sometimes known as different names in different sports. In baseball, for example, it is often called the Run Line, and in hockey, it's called the Puck Line.