All drivers at a minimum must make one pit stop during the race, and change between at least two different types of tyre (soft, medium or hard, unless it's raining in which case this rule doesn't apply, although you still have to stop but can use the same kind of tyre twice).

Soft tyres are theoretically the fastest but they degrade quickly, whereas hard tyres are theoretically the slowest, but last a lot longer.

So at the start of the race you will usually see a mix of strategies such as starting on soft tyres to try and gain an early advantage, at the cost of needing to pit sooner, or potentially pit more often than your rivals.

Alternatively you might start on the hard tyres and try to maintain a decent pace and stop fewer times, meaning you will gain places when those ahead have their pit stops.

In reality the strategies aren't quite so black and white as this, and of course your plan A might be invalidated if something unexpected happens like a safety car, rain or your rivals doing something you didn't anticipate that you either want to exploit or protect yourself from.

This is what makes up a large part of the fun for Formula 1 race, wheel to wheel racing and overtakes are great and they do happen but at a much lower frequency than most other racing series.

Historically they also used to refuel during pitstops, which provided another layer of strategy where the weight of the car would increase with more fuel, meaning you would drive slower but could go for longer. However this has been removed in favour of more complex tyre strategies instead, to improve safety (there are many examples of refuelling incidents).

Hope this helps you understand it!