Muscles work against eachother in pairs, as one pushes the other pulls against it in the opposite direction, otherwise neither would move. Its like a pulley system.
To use a simple example - bicep/tricep. As the bicep contracts it bends the elbow. What do you think would happen if there was no opposing force (tricep) to stop that movement? It would keep going! Same in the other direction, without a bicep your tricep would extend your arm untill the elbow bent the other way.
To avoid imbalances between your muscles, you need to train with a balance between different movement patterns.
I.e.
Bench is a Push because you push the weight away from you.
Rows are a Pull because you pull the weight towards you.
In that example, if you bench at the exclusion of rowing, the pushing muscles (chest, triceps, shoulders) are going to become stronger than the pulling muscles (back, traps, biceps).
If this happens, the pulling muscles are too weak to counteract the force exerted by the pushing muscles, which will result in either postural problems (shoulders being pulled forward and inwards by a chest that is too strong for the back to counteract), or simply an injury.
The same goes for every minor and major muscle group in the body. Posterior chain means the glutes, hams etc etc. Those are the muscles that work against the quads for example.
Hence a balance is important.
Periodization is how your program is going to change over time to:
1 - Achieve your goals.
2 - Keep things from becoming physically and mentally stagnant.
Savvy?
