The UEFA Champions League is UEFA's elite team competition with top soccer teams across Europe playing for the right to be crowned European champions. The tournament, then called the European Cup, began in 1955/56 with 16 teams taking part. It changed into the Champions League in 1992/93 and has expanded over the years with a total of 79 teams entering in 2019/20.
The group stage involves eight groups of four teams, with each team playing everyone else in that group home and away. The top two teams in each group move on to the round of 16 and the start of the knockout phase, while the third-placed team drops into the UEFA Europa League, a competition played by teams who didn't qualify for the Champions League. The round of 16, quarter-finals, and semi-finals are played over two legs, home and away, while the final is hosted by a stadium that is selected almost two years in advance.
This is how the 2021/22 Champions League groupstage looked like.