This is a type of fictitious - or 'inertial' - force. Imagine we have two frames of reference - for example, a moving car and a non-moving road. This is necessary for the majority of physics, as it makes clear various differences in behaviour between the two.
Imagine you are sitting in the car. When the car accelerates, you may feel a 'force' backwards, pushing you into your seat. Now swap so you are standing on the still- or 'intertial' - road, looking at the moving car. It's then evident that there is no force acting backwards on the person in the car - it's a fictional illusion to do with your reference point. The same is true for gravitational force.