Common games in the NFL tie breaking rules are games that teams have with common opponents. For example, let's say that Team A and Team B are tied and are both in the running for a wildcard spot in the playoffs. Looking at each of their 16-game schedules, they have each played Teams C, D, E, and F at some point during the season. These are the common games. Team A beat teams C and D, but lost to E and F, for a common games record of 2-2. Team B beat D, but lost to C, E, and F, giving them a record of 1-3. Team A wins the tie breaker and goes into the playoffs, and team B is finished.
In order for this particular tie breaker to be enforced, there needs to be a minimum of 4 games with common opponents between the two teams. If they don't have at least 4 common games, this tie breaker gets skipped.
For this tiebreaker, what is compared is the percentage record. For example, let's say two teams in different divisions are tied for a wildcard spot. When looking at games against common opponents, one team has a record of 3-1, and the other team has a record of 3-2, having played one of the common teams twice. The 3-1 team would win the tiebreaker, having a better percentage.