Equilibrium

Zero net force. That is the entire condition, and the AP Physics 1 CED words it as translational equilibrium being a configuration of forces such that the net force exerted on a system is zero.

Zero net force means zero acceleration, which is not the same as zero velocity. A skydiver at terminal speed, a car holding a steady speed on a straight road and a book resting on a table are all in equilibrium. Newton's first law is the same statement read backward: if the net force exerted on a system is zero, the velocity of that system remains constant.

In practice it is applied one axis at a time, $\sum F_x = 0$ and $\sum F_y = 0$, because forces can balance in one direction while staying unbalanced in another. The CED spells that out and adds that the system's velocity will change only in the direction of the unbalanced force. On an incline, this is why the axes are usually tilted to lie along and across the slope.

Torque is a separate condition. Rotational equilibrium means the net torque on the system is zero, and the CED notes that a system may exhibit rotational equilibrium without being in translational equilibrium, and vice versa. A ladder can have perfectly balanced forces and still start to tip.

Getting to $\sum F = 0$ starts with the diagram: see how to draw a free body diagram and how to find net force.