In a programming competition, you are given a few hours (the exact number depends on the competition) to solve several problems (again, the exact number depends on the competition). You have to write a program which solves the problem. You get points for solving the problems. The winner has the most points. Winners of coding competitions can win thousands of dollars in a single contest.
There are many sites which host online coding competitions. I picked two of the biggest contests.
USA Computing Olympiad is for middle and high school students in the United States. There are 4 contests every year. The top 24 students get a chance to compete to be on the US Coding Team for International Olympiad in Informatics (IOI).