Okay kiddo, have you ever been outside in a storm and seen how the rainwater flows on the ground? It doesn't just stay in one place, right? It moves around and tries to find its way to a lower spot. This movement of water is called "transient flow."
In civil engineering, "transient" means something that is temporary or not permanent - like the flow of water during a storm. When we design bridges, dams, or other structures, we need to make sure they can withstand this kind of temporary flow. We have to figure out how much water might flow during a storm and how fast it will move. Then we use special calculations and models to design our structures to survive that kind of transient flow.
So basically, transient in civil engineering means considering temporary changes in water flow and designing our structures to withstand them.