As long as a separate external force is not exerted upon something, it will remain as it is.

For example, if you are on a stationary train and it suddenly starts to move, you try to remain stationary, so you are pulled backwards in relation to the direction in which the train is moving. If a moving train stops suddenly, you try to keep moving in the direction in which the train was moving, so you are pulled in the direction in which the train was moving.

When a separate external force is exerted upon something, it starts to move in the direction of that new force. The amount of movement changes depending on the strength of the force applied and the weight of the thing. For example, strong force is necessary to push and try to move something heavy. If you try to bounce a heavy ball and a light ball, the light ball will bounce back more quickly even with the same amount of force.

When force is exerted upon something, the same amount of force is always returned in the opposite direction. For example, if you push a wall, the same amount of force as you exerted when you pushed it is returned by the wall.