Framing' is 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.
In a scene, when a character does or says something unusual for the circumstances, the scene partner may pick at that, to question it, to treat it as strange, to be skeptical of it.
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.