Logs
A log is the simplest kind of note: a line of text your app writes down as something happens — "user signed in", "payment failed: card declined", "cache rebuilt in 1.2s". If you have ever added a println to work out what your program was doing, you have written a log.
Logs are the running diary of your app. When something goes wrong on a real person's device, the logs are usually the first place you look.
Record a log
Each line finishes the moment it is written to the device — the same as traces and metrics — and is delivered later with no duplicates. That whole journey is on Device to dashboard.
Logs you already write
Your app almost certainly writes logs already, through a logging library. You do not have to rewrite any of that to get those lines into the same offline-safe buffer — and you can then reach into a device you cannot otherwise get to (a tester's phone, a CI simulator) and pull the logs off from a test. That is its own short how-to: Capturing.