It means that muscles get activated simultaneously in certain movements. Muscles don't get activated by themselves. Hence why all good kinesiologists and phy. therapists dislike the term 'isolation'. No muscle ever really contracts in isolation.
Many people set up their programs by training certain muscles on a certain day. However, they are training many more muscles than they think they are. For example, they will consider the bench press a chest exercise, then the next day train arms and shoulders. The fact is, they are training their arms and shoulders heavily while doing a bench press. They may be overusing certain muscles setting up a program that way. That is just one example, but the same concept can often pop up quite often in program design.
Setting up a program by training movements is considered superior for the average trainee. So instead of training by specific muscles, which is physiologically impossible, people train by movements. The body has several kinetic chains, made up of several muscles, that are involved with accomplishing all basic and complex movements of the body.
This is a large factor to consider when balancing your program. I could do on and on but I hope you get the basic idea. I don't want to be too wordy.