The word log has several meanings:

1. A thick piece of wood cut from a tree trunk or large branch. When you see a log lying in the forest, it's usually a section of trunk that's been cut or fallen naturally. People stack logs for firewood, build log cabins from them, or sit on them around a campfire. Before logs can become lumber at a sawmill, they're transported as whole logs, sometimes floated down rivers in huge rafts.
2. A detailed written record of events, activities, or observations. A ship's captain keeps a log (or logbook) noting the ship's position, weather conditions, and important events each day. Scientists maintain logs of their experiments, recording what they observe and when. Your teacher might keep a log of classroom activities. When you log something, you're writing it down in an organized record.
3. The act of entering to use a computer system, website, or app. When you log in or log on to a computer, you're entering your username and password to access your account. You log out or log off when you're finished.