When body is at rest or in uniform motion gravitational force act on it. Forces like frictional force, viscous force act on body.
When object is at rest or in uniform motion, resultant force of various external force acting on it is zero.
Consider book at rest on horizontal surface following force act on it.
$(1)$ Weight of body $(W)$ in downward direction.
$(2)$ Normal force ($R$ or $N$) in upward direction.
Here book is at rest hence, $\mathrm{W}=\mathrm{R}$.
Effect of both force cancel out each other. Hence, book is at rest.
Actually it should be like this, considering book at rest by Newton's first law of motion resultant of external force acting on body must be zero. This suggest that normal force and weight should be equal and in opposite direction.