A round robin is a scheduling system where three or more teams are in a tournament or playoff. Each of the teams involved play each other an equal number of times. Sometimes they play each other twice - home-and-home - or if time is short only once.
This is opposed to the single elimination system which is used in the Stanley Cup playoffs, for example, where teams play only one opponent at a time.
The round robin system is used in, for example, the Memorial Cup and in the preliminary round of the Olympics. In the Olympics the leading teams play single elimination sudden-death games after the round-robin is finished.
Some tournaments also use what is called a modified round robin format. Usually in this format teams are paired off in two groups and the teams from one group only play teams in the other group. This format is sometimes used to give teams one additional game. It is usually used for 6-team (more so) or 8-team (less frequently) tournaments where the teams are grouped by their league.
This format has been used in the Alberta Junior C Playoffs since at least the 2012 tournament. The Noralta Junior Hockey League has 3 teams in the tournament and the Calgary Junior C Hockey League has 3 teams in the tournament and the teams each play 3 inter-pool games. The first place teams from one bracket plays the second place teams in the other bracket in the semifinals. The semifinal winners meet in the Provincial championship game.