In golf, par is the average number of times an excellent player needs to hit the ball, either for one particular hole or for the entire course. You might describe the fourth hole, for example, as being par five.
If a golf course is par sixteen, it takes a really good golfer about sixteen strokes to get her ball into the final hole. From the sports meaning of par, it's come to also mean "average" or "equivalent," especially in phrases like "under par," or "worse than expected," and "par for the course," or "what is normal given the circumstances." This definition actually came first, in the form of the Latin par, "equal."