F= force of friction
N=Normal force =mxg here g=acceleration due to gravity or strength of gravity
u= coefficient of friction
Normal force is a perpendicular force applied to the surface to the body. It is acted on the body by surface under the influence of gravity on the body in the downward direction.
Gravity is the downward acceleration experienced by anybody on the surface of any planet. This is responsible for weight experienced by the body. On a flat surface weight and Normal are opposite in direction. The surface in contact produces frictional force.
Friction is opposite to the direction of push applied on the body to make it move.
If gravity is very strong it becomes harder to slide a body on a surface because of increased force of friction.