When you are mountain biking over rough ground, it is not uncommon to get a dented rim. This can be caused by rocks, tree stumps and potholes. If the impact was big enough to give you a pinch flat, it is likely to cause problems to your ride and will need to be repaired or replaced. If it was less severe, it is unlikely to cause ride problems but will certainly affect the braking.
It is common to feel a pulse on the brake levers when the brake pads hit the area where the dent is. Sadly, this makes it more likely that you will get a blowout of the tire. Sometimes, a flat spot is created on the rim and so it is no longer perfectly round. If this happens, it is likely that you will need a replacement rim or wheel. The good news is that you can repair minor dents yourself and it takes just 10 minutes or so.
You have to start by removing the damaged wheel and taking off the tire and the tube. Then, take a flat wrench (e.g. a cone wrench) and hold it lengthwise against the outside of the rim where the dent is located. Then use a small adjustable wrench (or you can use vise grips) to slowly work your way along the dent and pry the rim outwards towards the flat wrench. Don’t worry if you cannot get it perfectly smooth, you just need to reduce it in size enough for it to be unnoticeable when you are riding. Check to see if any spokes in the area are broken and if all is well, put the tire and tube back on.