When your essay about French cooking starts describing a childhood trip to Disneyland, it's taken a digression — it's strayed from the main topic.
"But I digress" is a phrase often used by people when they realize they're no longer "on-topic." A digression is like a tangent, only digression often describes speech patterns, whereas tangent comes to us from mathematics. Another trick to remembering the meaning of digression is its relationship to the word progression. A progression is a series of ideas which proceeds in the same direction; a digression, logically enough, is an idea that goes off in another direction.