Usually: The wear indicator on the brake pads.
But it can also be: Glazed pads, cheap pads, or just moisture on the discs first thing on a damp morning, which stops after the first few stops.
Squealing when you brake
Most of the time this is the wear indicator doing exactly what it is supposed to do. Brake pads have a small metal tab built into them, and once the friction material wears down far enough, that tab starts touching the disc and makes a squeal. It is a warning, not a breakdown. It usually means the pads are due, not that anything has failed.
It is not always that, though. A few other things sound similar. Glazed pads — where the surface has hardened from heat or from a lot of light braking — can squeal even with plenty of material left. Cheaper aftermarket pads sometimes squeal from new because of the friction compound they use, with no wear issue at all. And if it is a damp or cold morning and the squeal goes away after the first few stops, that is often just moisture sitting on the discs overnight. That one is harmless and costs nothing to fix.
How to tell which one it is
A few things to check yourself. First, does it stop after the first handful of stops on a cold morning? If yes, that points to moisture, not wear. Second, does it happen every time you brake, at any speed, warm or cold, day after day? That pattern points more toward the wear indicator or glazing. Third, if you can safely glance at a front wheel with the wheel off or through the spokes, look at how much pad material is left between the metal backing plate and the disc — if it looks thin, that is your answer. If you are not comfortable pulling a wheel, this is a two-minute look for us.
Is it safe to keep driving
Generally yes, for a short while. A squeal on its own is a warning sound, not a sign of brake failure. But it is worth getting looked at soon rather than waiting, because worn pads that go too long can start to score the discs, and that turns a pad replacement into a pad-and-disc replacement, which costs more.
What fixing it usually involves
If it is pad wear, it is a brake pad replacement, sometimes with a disc resurface or replacement if they have worn unevenly. If it is glazing, sometimes the pads can be dealt with without full replacement, sometimes not — depends how far along it is. Cost varies with the vehicle and how much material is left, so it is not something we can quote without seeing it. If you want, bring it in and we will pull a wheel and tell you straight what it is before anything gets done.