Why times runs slowy when gravity is high?
Strong gravity means that spacetime is curved which means that time passes more slowly.
Einstein's field equations show that mass, energy and momentum cause four dimensional spacetime to curve. Curved spacetime causes gravity. It also causes time to pass slower.
The equation which gives the factor by which time slows due to a mass
Notice that if the mass is zero or the radius is very large, the factor is one and time runs at its fastest. Now, as the mass gets bigger and the radius gets smaller the factor gets smaller and time slows.
Also notice that there is a special value of the radius where the factor is zero which means that time stops.