A follower force "follows" the geometry as it deflects. It is applicable to geometrically nonlinear problems where deflection is relatively large.
For example, consider a cantilever beam with large deflection. Do you want the initial load direction to remain in the same direction during the entire analysis, as it deforms? Or do you want the load to remain normal to the tip as it deflections (the load direction is continuously updated based on the deflected geometry). The second case is where the force "follows" the geometry.
