In a nutshell, the study of measuring time. Most textbooks start with the stars and how we can use them for that purpose, including of course our own. Solar clocks, water clocks, you name it and it existed before the modern clock was invented.
Turns out that timekeeping was crucial for navigation so in a sense, making clocks better became a precondition of navigating the globe and a lot of advancements in (micro)mechanics started there.
Of course, these days we keep all but perfect time and position using atoms and satellites and software, but a lot of people are attracted to these marvelous mechanical devices that measure time. So learning about (mostly mechanical) watches and clocks, what makes them tick, how to maintain them and how to repair them is usually the topic of discussion for horology forums.
It’s a very deep rabbit hole. But a fun one.
I started going down it because I like tinkering, doing fine motoric stuff but found spending money on Lego sets got boring and I ran out of space to display them. Watches are much more space efficient and when done with them, you can sell them :)
It’s a cult and you just joined it ;-)
There’s a ton of reading on all these topics. If you’re interested in browsing what the actual process of tinkering on watches and clocks entails, archive.org has scans of relevant books, like https://archive.org/details/practicalwatchre0000deca and https://archive.org/details/watchclockmaking0000wjga. If lending your eyeballs to the algorithm is more your thing, https://www.youtube.com/watch?v=dJxnP0qA2hU&list=PL7q4FTKNfQj-bcGk0JfUXioeq6KNtyhVA is a neat playlist.
There’s a bunch of videos on probably one of the most interesting devices in history, the Harrison H4 marine chronometer. Whether it was the first or the best is probably less important than that there is a juicy story about this nerd that comes out of nowhere and officials refusing to hand over the large pot of money that the British promised to whoever could make a clock precise enough to be used on a ship for navigation.
An actual modern watch making hero is George Daniels, who essentially built watches by hand from scratch - bars of gold, steel, brass entered his shop and working watches came out (at the tune of maybe one a year). A bunch of documentaries were made about his life and work and I think they’re all quite cool.
Lemme know if you need more :P