Hey, I literally JUST had that question pop up into my head and decided to test something out since I didn't know either.What I found out was basically that the "last activity" means when YOU last had contact with them. For example, if you haven't talked to someone for five days, it'll say that the last activity you had with them was five days ago. Try it with the Echo Sound Test system. If you talk to them and then immediately check "last activity," it'll tell you last activity: 2 seconds ago.
Other people asked questions on various topics, and are still waiting for answer. Would be great if you can take a sec and answer them