2 main things cause brakes to grind. Brakes grinding can mean nothing or it can mean your brakes have a major safety issue, here is how I work out which is which.
1.Brake pad has run out of material. Either through normal use or because the brake pads are wearing unevenly.
2.Debris caught between the metal brake disc cover
If you have number 1……
Its a major issue to braking performance so you should get it checked out immediately. When you need your brakes the most ie when you need to brake hard, the worse performance you will get from your brakes. In other words, you wont realise how bad the problem is until you need your brakes for an emergency.
If you have number 2….
you should also get it checked out. In my experience this usually works itself out. But if you are not sure, get it checked. If you are slightly mechanically inclined you can check the disc yourself. Stones and debris usually get caught on the edges of metal cover that is on the inside on the brake disc. If you are lucky, you may be able to find the problem and remove it with your fingers or a screwdriver but be careful not to push the piece further in! Reversing may also help. If in doubt, speak to a professional.
In summary, brakes grinding is something we all experience at one time or another, if you are in any doubt with regards to if your brakes grinding is safety problem, find a mechanic.