Remember that one group project in school where you did ALL of the work and your partner just kinda watched as you carried the team (...maybe you were on the other side... we’ve all been there). Your body does that too. You have a dominant side. A side you write with and pick things up with. A side that always pushes off starting the run or is a little stronger on the barbell. 

A great way to fight the over-development of this dominant side is to train unilaterally, or train exercises that use one side of the body or one limb at a time (i.e. single arm, single leg).

When introduced to barbell work (back squat, clean, bench press) the dominant side of the body or dominant limb may compensate for the weaker one, leading to imbalances in that movement and in training. Without working on unilateral movements, the weaker side might not catch up strength-wise, thus further widening the gap / imbalance between the two sides. If left alone this may lead to possible injuries...