The NFL will go to a modified sudden-death-overtime system for the playoffs to determine a winner in the event of a tie at the end of regulation. The system guarantees that each team gets a possession in overtime unless the team that receives the opening kickoff scores a touchdown on its first possession.
If the team receiving the opening kickoff doesn't score a touchdown, then each team will have at least one possession to determine a winner. If the score is still tied after each team has a possession, then the game will continue until one team scores.
If, for instance, each team scores a field goal on its initial possession, the game would continue until one team scored next.
The system was enacted to avoid giving an unfair advantage to the team that receives the opening kickoff in overtime.