If you see a Lifesaver footnote in the PoTG screen, this is intended to feature situations where one player is able to save another player (or players) from imminent death. For example: If Reinhardt has an enemy pinned, and that enemy's teammate stuns or kills Reinhardt, the teammate would be awarded a large amount of "Lifesaver" score. If Mercy has 5 of her teammates downed, and casts her ultimate, the same situation would apply and Mercy would be awarded a large amount of "Lifesaver" score as well.
From the Overwatch wiki:
The player who has the highest score in any one of these four
categories at the end of the match will receive the Play of the Game.
If the winning Play of the Game category is Lifesaver, Sharpshooter,
or Shutdown then the category name will be called out during the
highlight intro. If the winning category is High Score, however, no
callout will be provided.