Affect refers to the emotional state right in the here-and-now. The ups and downs of a person's affect, as they average out over a longer period of time (say weeks or months) are what we call their Mood.

A helpful analogy is that WEATHER is to CLIMATE as AFFECT is to MOOD.

Affect storms are common and unhelpful for therapy, as they effectively stop, or invalidate, mentalizing.
Affect Regulation is a capacity that therapy is designed to increase.