If we're talking about science fiction, and you suddenly go off on a long tangent about the cost of grape soda, you digress. When a person digresses, they stray from the topic.
It's easy to understand why you digressed from the main topic. You were incredibly excited, speeding on caffeine, and, let's face it, you've never been at a loss for words. Don't get me wrong: I love your wanderings — how you drift from one topic to the next, letting your mind explore. The problem was that the meeting was running late and there was no time to let you digress. I had to pull you back to the main topic. I didn't want our potential new partners to get the idea that these digressions were typical.