The brakes in your car work hydraulically, which means that when you press that middle pedal (assuming you've got a manual-gearbox car...), some fluid is pushed through various hoses and pipes, so that at each corner of the car a brake is applied.
Your brake fluid is topped up via the master cylinder, which sits on the driver's side of the engine bay, below the windscreen. Most modern cars have a sensor which tells you that the level is low; your car shouldn’t get through brake fluid as such, but as the brakes wear, the level will drop a bit so you might have to top it up very occasionally. However, if you're constantly having to top it up there's a problem – one that needs investigating urgently.
Every couple of years or so, your car's brake fluid needs to be replaced
Most cars are built in such a way that the brake fluid never needs to be topped up, and because the sensor is always happy that the system is functioning correctly, you just carry on driving, oblivious. But danger lurks ahead if this sounds like you, because every couple of years or so, your car's brake fluid needs to be replaced.
Brake fluid is what's called hygroscopic, which means that it absorbs water from the atmosphere. Over time, your brake fluid becomes more and more dilute, which reduces its effectiveness. Not only does it compress more easily (which means you get a spongy pedal), but it also lowers the boiling point of the fluid. If you use your brakes a lot, the result of all this is that you could end up with the brake fluid boiling – so you potentially lose your stopping power altogether.
It's possible to replace your car’s brake fluid on a DIY basis and the materials on their own are just a few pounds. But unless you really know what you're doing we'd suggest you enlist the help of a qualified mechanic, because if you get things wrong, you could end up losing your brakes altogether. You should expect to pay £50-100 to have the work done.