Okay, picture a long tube (like a drinking straw) that carries a bunch of little balls through it. These little balls are like bundles of instructions that the computer has to carry out really quickly. Sometimes, the computer might realize that it made a mistake or there's a problem with one of the instructions.
So, instead of letting all the balls go through the tube and finish the sequence, the computer stops the balls and forces them out of the tube. This is called a pipeline flush. It's like stopping a train halfway through its journey because there's a problem with the track ahead, instead of letting it go all the way and then trying to fix it.
By doing this, the computer can quickly fix the problem without waiting for all the instructions to finish and possibly cause bigger problems (like crashing). Once the problem is fixed, new little balls with fresh sequences of instructions are put into the tube and the computer can continue carrying them out.
So, a pipeline flush is like hitting a restart button on a bunch of instructions in order to quickly correct any errors or problems.