Framing means to communicate to your scene partner that you noticed that they said or did something potentially unusual within the context of the base reality. When you frame, you highlight or underline what you find to be unusual so that your scene partner is aware of it and you both can treat it as the jumping point for a game.
In a scene, when a character does or says something unusual for the circumstances, the scene partner may react to it in many ways to make clear what may be considered as unusual, by picking at that, questioning it, treating it as something weird, being skeptical of it, or being annoyed by it, among other truthful reactions.
Sometimes your scene partner doesn’t realize that they have said or done something unusual. This allows both improvisors to agree that on the first unusual thing in the scene.
"Framing means letting your scene partner know that you feel that they have said or done something unusual within the context of the base reality. When you frame, you highlight or underline the unusual so that it stands out to your scene partner."
- – Upright Citizens Brigade Comedy Improvisation Manual