The washer is one of the most common causes of leakage in bathroom taps. It's also one of the easiest to learn how to fix. Because it has an important function inside the valve, it could eventually wear out after long use, or it could simply be that it was made of shoddy material. Either way, you could fix it. Here's how.
Fortunately for you, you needn't learn a whole lot of plumbing schematics to fix it. You're only going to need two things: a slotted screwdriver, and an adjustable spanner. When you're ready to start, turn the stopcock so as to cut the supply of water. Next, using the screwdriver, carefully unscrew the valve and twist it; then, by using the adjustable spanner, remove it. Overturn the open part of the headgear so that the water inside would drain.

Expose the insides of the headgear to find the jumper. Attached to that is the washer. Replace the washer with a new one and put everything back together. That's it. As easy as that, you could fix leaking faucets. Lastly, turn the stopcock back on and check if the water is still escaping. If so, then the problem must not be on the washer.

Keep in mind that leaking faucets would seem nothing more than a nuisance. But those few drops, overnight, over the week, could easily add up to your water bill. It's always best to fix a leak right away when you see it.