@kira well, primarily navigational information, but really anything.

Originally, the term "log book" (also log file) comes from a navigational device called a log. It's a piece of wood with a string on it. You throw it over the side and measure how long it takes to extend fully. That's how fast you are going. Do that regularily and record it in a special book (uh, because of what goes in it just call that the "log book"). If you know how fast you've been going for how long, then you can reckon (reason) where you are. kinda. That's called "dead reckoning". Before they invented decent clocks, you could measure your latitude easy enough, but you had to use dead reckoning to know/guess how far east/west you where.

Also, anything else relevant to sailing. For example, the weather, what direction you are heading, what sails you have set, things that you have past or are trying to avoid. Anything that has broken or you've fixed. Anything that might be important to know happened on a certain date, such as an injury. Sometimes notes on stuff (such as the stocktake) and personal reflections. So sometimes it's a diary. If in my telling, there is a time to the minute for something, that event was certainly recorded in the log book. On this trip, I just navigated the least interesting way, gps and charts on my phone. I still wrote down the lat:long at noon, so that I could calculate a daily run*. If you doing something more interesting, such taking bearings off landmarks, or celestial navigation, or even just lat:long and marking it on a paper chart, all that stuff would be in the log book too.

* traditionally, a daily run is measured noon to noon, because if you are doing celestial navigation, that's the time you most accurately know where you are.