Variable mass
Definition
Variable mass means the mass of the moving object itself changes over time — most famously a rocket burning fuel, which gets lighter as it accelerates.
Technical definition
Newton's second law generalizes to F = dp/dt = m·dv/dt + v·dm/dt. For a rocket ejecting mass at exhaust velocity v_e, thrust is F = v_e·|dm/dt|, leading to the Tsiolkovsky rocket equation.
Example
A rocket burning 100 kg of fuel per second with exhaust velocity 3000 m/s produces 300,000 N of thrust — before even accounting for gravity or drag.
Common confusion
You cannot just use F = ma with the starting mass for a rocket. Because mass leaves the system, momentum accounting must include the ejected fuel — that is why rockets can accelerate in empty space, pushing against their own exhaust.