In sports and (combative) games, power play refers to either tactics to increase a player's or team's power or influence, or situations where one team has an advantage, e.g. due to penalties reducing the number of active players of the opposite team.
In human sexuality, power play refers to the taking of complementary roles that set two or more participants at different levels of power. Such an agreed unequality of power is the basis for all forms of BDSM.
Sexual power play
In abstract and neutral terms, the players in sexual power play are referred to as top and bottom. The top(s) are using their power to control and/or punish the bottom(s), and both enjoy the power imbalance that is the core of such play.
Typical BDSM play starts with a non-physical power imbalance that can be expressed e.g. verbally, by tone of voice, by posture and/or clothing. The top usually takes the more active role and gives commands and orders to the bottom, who submits and is expected or made to behave generally passive. Then the top typically takes actions that deepen the power imbalance and take it to a physical level, e.g. by binding the bottom.
In the spankophile community, we find the same kind of power play, but members of the spanko subculture often prefer less extreme power differences -- e.g. not Master/slave but perhaps teacher/student or parent/adult child roleplay. In these, the spankee is not always acting submissive from the start but can be acting non-cooperatively and bratty, perhaps even having to be physically overpowered after some struggle as part of the play. We also find forms of spanking among two equals without any power imbalance, including play spanking, sensation play, sensual and erotic spanking, and topping from the bottom is less frowned upon.