On average, mountain bike tires are rated for 30—60 PSI. And often, I find people are taking full advantage of the 60 psi pressure. This is a problem, especially for new and intermediate riders.
At higher pressure, the tires will roll faster. This is good. But, when you make contact with rocks and bumps on the trail, the tires will tend to bounce and skip rather than roll over these objects. This is bad.
At lower pressure (30-35psi), the tires will roll slower on the trails. But, when you hit an object, the tire will almost “pancake” in shape as it absorbs the rocks and other bumps on the trails. This is good.
As a new or intermediate cyclist, check you tire pressure before every ride. Believe me, it makes a big difference.
How low can you go?
If you go below 30 PSI, you start running the risk of getting pinch flats. I don’t recommend going lower unless you have tubeless tires.
**** Always follow the tire manufacturer recommendation for tire pressure.
