What does a negative force mean?

Hint: Force is a vector quantity and depending on the direction where we use force. Consider that we are implementing force to an object, but we cannot migrate that object in the force's direction. In this case, the object is implementing a reaction force that is larger than the applied force. Therefore, the remaining force is opposing.

Complete answer:
A negative force is an opposing force acting on an object against the implemented force. The concerned force here is positive and aimed in the right direction, whereas an opposing force appears to act in the left. An instance of negative force is deceleration, where the velocity of a moving object reduces due to gravitational force.
If the object being operated upon is stationary, the total of all the forces is zero. So force A must be representing downwards, not upwards. A 'negative' force is the opposite of what you choose to be the positive direction.
Magnitude of a force is never negative but direction can be negative that is opposite direction. When a body goes on a surface, the friction force between the body and the surface is in a direction reverse to the body's motion, so the work performed by the friction force is negative. The negative sign denotes only the direction.
Force is a vector quantity so that that force can be opposing, depending on direction and magnitude. If the implemented force is in the reverse direction to the displacement of the traveling object, then the force will be opposing or negative.
Example: A body is moving and a force $20$ N acts on this body, and a friction force of magnitude $28$ N acts on it then negative force is $-28$ N. This is in the opposite direction.

Note: Force can be positive or negative. If there is negative acceleration, the force will be negative. Usually, Force working toward the right is considered as positive. The force's magnitude is always a positive amount. The force working towards the left is considered negative. Force implemented on an object is directly proportionate to the mass and object's acceleration.