An object will remain at rest or in a state of uniform motion unless it is acted on by a resultant force.
Inertia:
A property of matter by which it continues in its existing state of rest or uniform motion in a straight line, unless that state is changed by an external force.
more mass = more inertia
Therefore, heavy objects are more difficult to get moving but once it moves it’s also more difficult to make it stop or change direction.
In both cases you will need a larger force than with a smaller object.
Example: a seat-belt in a car stops inertia from smashing you into the wind-shield in a car crash.
The Normal Force:
The force exerted by a surface on an object in contact with it.
The Frictional Force:
the force that opposes the motion of an object in contact with a surface and it acts parallel to the surface the object is in contact with.
For a body of constant mass, its acceleration is directly proportional to the resultant force applied to it.
$$\frac{N-F}{a}$$
When two bodies interact, the forces they exert on each other are equal in magniture and opposite in direction.