What does the equation F − ma = 0 mean in d’Alembert’s principle?

In d’Alembert’s principle, the equation $F-ma=0$ represents an alternative formulation of Newton’s second law of motion. By rearranging the standard expression $F=ma$ into this form, the principle effectively reduces a problem in dynamics to one of statics.1

In this context, the equation signifies that a body is in a state of equilibrium when the real force $F$ acting upon it is combined with the negative of the product of its mass and acceleration ($-ma$). This term, $-ma$, is referred to as a fictitious force, inertial force, or a reversed effective force.1

By treating a moving body as if it were in equilibrium, engineers and physicists can simplify the analysis of forces and stress in complex systems, such as machine components, because unknown forces are often more easily determined in static conditions than in moving ones.1