A plié is defined as:
Bent, bending. A bending of the knee or knees. This is an exercise to render the joints and muscles soft and pliable and the tendons flexible and elastic, and to develop a sense of balance. There are two principal pliés: grand plié or full bending of the knees (the knees should be bent until the thighs are horizontal) and demi-plié or half-bending of the knee.
Pretty much every class starts with pliés and they’re usually the first thing we learn as ballet dancers, no matter what level we start at.
Suzanne Farrell said, “Plié is the first thing you learn and the last thing you master.”