How do you explain to your almost pre school kid (4 yrs old going 5) what "discipline" & why we, as a parent, need to discipline them?

Explaining the word discipline at this age can only be done by sighting examples each time he/she behaves badly with you or anyone. If the child misbehaves with you and doesn't want to understand, let that be, and next time when he/she asks for something you answer or behave in the same way as he/she may have done. And when the kid feels bad, tell him that this is how you felt when he misbehaved or didn't listen. Be gentle in conveying the message to the kid and when you think he has got the point, hug him. With my son, when omitting like this happens, we when sort out the things, make a deal that when I misbehave, he will remind me that I am not supposed to behave like that and visa-versa. Eventually, kid will understand discipline, and you will not have to define the word categorically.