A chemical equilibrium is a chemical reaction that is reacting in both directions (forward and backwards) at the same time, and at the same rate. Nothing appears to change (macroscopically - on the large scale), but both reactions are dynamic microscopically.
7.1 Le Chatelier's principle allows us to predict the behaviour of an equilibrium.
The equilibrium law
17.1 Equilibrium calculations, like mole calculations, need lots of practice.