I'm not a doctor, nor do I care to be one, so this is in "Laymen's Terms" as it's called and probably not a 100% scientifically correct explaination. When your skeletal muscles contract (the muscles connected to bone), in order to do so, the brain has to send a signal to those muscles to contract. That signal then travels somehow or another through the nervous system and eventually the signal reaches the desired muscle, and through another process (the sliding filament theory), the muscle contracts. Looking at the diagram to the right, you can see that the signal is collected by dendrites, passed along via the axon and attaches to the muscle at the neuromuscular junction. If you're confused, all you have to do is literally read the diagram and it shows what I just said.
So, why should you give a shit about this? Although most of the signals sent by our brain are unconscious, when lifting weights you can actually control which muscle the brain sends the signal to! Cool huh? Unfortunately, most of us just go through the motions and have no idea how to control these signals! Let's use an example of an experience you may have had lifting where the wrong signal was sent. You are doing a Dumbbell Chest Press. Your form is PERFECT, however, you can't seem to feel anything except your shoulders! Oh no. Well, there are a handful of reasons this is the case (please see Stretching for a starting point), and one of the reason may simply be because you aren't thinking about the muscle. Seriously, it can be that easy. When you think about the muscle, you are telling your brain that is the muscle you want to contract and it usually listens to you, but what if it doesn't? Again, there could be a few reasons. The reason I want to focus on is that your brain probably has no idea how to send a signal to that part of the body. If your form is perfect, you're thinking about the muscle, and you still can't feel it, you probably need to activate the muscle. Activating a muscle is relatively easy. You need to pick an exercise that is extremely specific to the muscle you are trying to contract i.e. a Pec Fly to activate the chest, and REALLY SQUEEZE that muscle during the activation exercise. Do make sure you're not overactivating the muscle to the point where it gets fatigued, you pretty much just want to feel that muscle flex. What you are doing is helping the brain find that pathway and "giving it permission" to use it. Then, try the main exercise again and let the magic happen! This is what I call "Mind to Muscle Connection."