Concurrency is a quite difficult concept to fully grasp immediately, so let's consider a quick metaphor, in order to make concurrency and its differences from parallelism easier to understand.
Although some neuroscientists might disagree, let's briefly assume that different parts of the human brain are responsible for performing separate, exclusive body part actions and activities. For example, the left hemisphere of the brain controls the right side of the body, and hence, the right hand (and vice versa); or, one part of the brain might be responsible for writing, while another solely processes speaking.
Now, let's consider the first example, specifically. If you want to move your left hand, the right side of your brain ...