Situations are aspects that your character discovers or creates. From a player perspective, situations can be thought of as a commodity that the player can create and spend to influence the story. Situations have a few properties that may or may not be different from a normal aspect:
If the character deliberately creates or discovers (if he/she/it was the first to do so) a situation for use later, it may be invoked for free once. If the check to create the situation is a critical success, it may be invoked twice for free.
If the situation is a one-time event (surprising a character, etc.) then after it is invoked it is safe to erase the situation from your sheet.
Situations can be forgotten without harm. If you delete an aspect part of that entity is gone, however situations are meant to be forgotten after they have played their role in the storyline.