American odds
American odds express a price as a number above or below zero, where a positive number is the profit on a 100 stake and a negative number is the stake required to profit 100.
In plain English
A price of +150 means risking 100 to win 150. A price of -150 means risking 150 to win 100. The sign tells you which side of even money you are on, and the magnitude tells you how far.
Example
At +200 a 50 stake returns 100 profit. At -200 a 100 stake returns 50 profit.
Why it matters
Almost every American sportsbook quotes this way, so reading it fluently is the difference between comparing prices and guessing at them.