Resurrection, or Resurrect, is an item you can get in the game Running Fred. It is also counted as a one-time use only skill.
This is definitely one of the most powerful items in the game. In Adventure Mode levels, if you are killed by Grimmy, traps or obstacles, a popup will ask you if you want to use this item. (Note that if you still have extra lives after the death, this popup won't be initiated.) If you answered yes, you would be given an extra life immediately, starting from the last checkpoint.
As for the Infinite Mode, this can still give you an extra life after your death. But magically, if you resurrect yourself, this doesn't count as a "death", and you can continue to play the level without the record and your Skullies gone as well. This can be extremely useful, if you are trying to set a new high record, or you want to have the record high enough to unlock higher difficulties in Infinite Mode.
This Resurrection item looks like a heart with folding, white, angelic wings spread out behind it.
You can get these in several locations, through the shop or gameplay. You can buy this through the shop if you have enough Skullies, and you can get them through Mystery Boxes before the update. If you are so lucky that you can get a resurrection in the Mystery Box, they usually come alone or in pack of three, which requires luck.