An apple falls from the tree due to the law of gravity, propounded by the Newton. According to the law of gravity, earth has the power to attract everything towards it. In simple words, what goes up comes down due to this force.
For example- when we throw a ball in the air, it comes back on the floor/ down due to the law of gravity. It is also called Newton's universal gravitational attraction force.
According to the gravitational law, the entire universe has gravitational force and due to this force the two objects which are present in the universe catches each other. This force or attraction is known as gravity or gravitational force.