A handicap index is a numerical measure of a golfer's potential that is used to enable players of varying abilities to compete against one another. A handicap is intended to reflect a player's potential or "average best", not a player's overall average score; it is calculated based on the best 8 of the player's last 20 rounds, with a minimum of 5 rounds required before a handicap index is determined.
Your handicap index is the starting point. Whenever you play competitively, you need to calculate both your and your opponent's course handicap, which will tell you what your handicap is for the course you are playing. The easiest way to do this is to use the Course Handicap calculator in the Golf Canada App, which you can download for free from the Apple App Store or Google Play.
Suppose Sally has a handicap index of 17.9 and she is playing a match against Julie, who has a handicap index of 22.4. They are playing at Lakeview Golf Course in Mississauga, both playing from the red tees.
Sally's course handicap for the red tees at Lakeview: 16
Julie's course handicap for the red tees at Lakeview: 20
So Sally will give Julie 4 strokes, which translates to 1 stroke on each of the 4 most difficult holes, handicap number 1 through 4.