Line
The line is the handicap or threshold a market is set at, such as a point spread or a total.
In plain English
Line and price are different things and move independently. A spread can stay at -3 while its price moves from -110 to -125, and it can move from -3 to -3.5 while the price returns to -110.
Example
A team is -3 at -110. Later the same market reads -3.5 at -105. Both changed.
Why it matters
Confusing the two makes market movement unreadable, and half a point on a key number can matter more than a large price change.