Normal Force:Normal force refers to the support force exerted by a surface that is perpendicular (normal) to that surface. For example, when standing on the ground, your weight is balanced by the normal force pushing up from the ground.
Frictional Force: Frictional force refers to resistance encountered when two objects move against each other. In this context, frictionless means there is no frictional force acting between surfaces.
Equilibrium:Equilibrium occurs when all forces acting on an object balance out and there is no net acceleration. In this case, if block A isn't accelerating vertically, then the magnitude of gravitational force pulling it down must be balanced by an equal magnitude of tension pulling it up.