Gravity is a force which changes motion.
Gravity can only act towards a mass, in most cases (on Earth) this is simply in the 'downwards' direction.
If you throw a ball in the air, its motion is initially 'up'. Gravity pulls on the ball, slowing its motion by 9.8m per second, per second. So if the ball initially has a speed of motion of 9.8m/s then it will only move upwards for 1 second - gravity has overcome the ball's motion.
Then the ball will stop (momentarily) in the air before returning to your hand. Its rate of descent is given by the force of gravity - again 9.8 (m/s)/s.
Forces are given by Newton's Second Law of Motion: F=ma, here the 'a' is acceleration due to gravity (9.8 (m/s)/s); m is the mass of the object the force is acting on.
So, for example, vector motion is described by `r=r_0+v_ot+1/2at^2` . If there is a y-component to the motion (up or down), then gravity will make the overall motion 'more downwards'.