Usually: The pads are gone and metal is running on metal.
But it can also be: A stone caught in the brake dust shield, which sounds similar but comes and goes.
Grinding when you brake
Stop putting this one off. Grinding when you brake usually means the pad material is gone and you're down to bare metal on metal. That metal is either the backing plate of the pad or the caliper hardware, and it's grinding directly against the disc every time you press the pedal. Worth dealing with now rather than later.
There's a second, much cheaper possibility. A small stone can get caught between the disc and the dust shield, which is the thin metal plate behind the disc that isn't part of the braking system at all. A stone in there makes a similar grinding or scraping sound. The way to tell them apart: the pad problem grinds every time you brake, and it usually gets a bit louder over a few days as more material wears away. A stone tends to come and go. It might grind for a block, then stop, then start again, and it often changes with speed rather than with braking. If you can pull over and spin the wheel by hand with the car in neutral, listen for the same noise without touching the brake at all. If it grinds during the spin, that points to the stone. If it's silent until you press the pedal, that points to the pads.
Either way, is it safe to keep driving? For a stone, yes, at least in the short term, though it's still worth getting looked at since a stone can scratch the disc surface over time. For worn-out pads, no. Once you're metal on metal, every stop is cutting a little groove into the disc surface, and your stopping distance is getting longer with every mile. This is not a wait-a-few-weeks noise.
Fixing worn pads by themselves is a straightforward job. But if you've been driving on the grinding for a while, the discs have likely picked up scoring or wear from the metal contact, and at that point it's a pad and disc job rather than just pads. That's not us upselling you, it's just what metal on metal does to a rotor. We'll pull the wheel and take a look before we say which one it is, and we can walk you through what we're seeing.
If it turns out to be a stone, the fix is often as simple as removing it and checking the shield isn't bent or rubbing. Either way, it's worth booking it in rather than waiting to see how bad it gets.