Both a solar-powered digital watch and analog version rely on solar cells that are normally arranged around the watch face and absorb light throughout the day. Most solar-powered watches come already charged out of the box, but if you need to charge it yourself, that is easy enough, and there should be instructions that come with the watch.
The good news is that, unlike most things we use daily such as smartphones, tablets, or laptops, you do not need to charge your solar watch as often, and many watches can provide up to 10 months of power from a single charge.