Consider a simple chemical system including just two compounds, A and B:
This just says that if you mix A and B, A will react to become B, and B will react to become A. This equation also states that one molecule of A reacts to give one molecule of B. This will be an important part of the concept. A is just turning into B, and vice versa. No other substances are involved (presumably). Associated with this system are two quantities, Q, the reaction quotient, and K, the equilibrium constant.
It is important to understand the distinction between Q and K.
Q is a quantity that changes as a reaction system approaches equilibrium.
K is the numerical value of Q at the "end" of the reaction, when equilibrium is reached.