Basically this is surgical plumbing. Imagine you have a sink on the 2nd floor of your house that is not draining because a pipe somewhere between the 2nd floor and the basement is blocked and clogged up. You don't want to tear up the floors and walls to fix the clogged pipe so you attach a new pipe to drain the sink upstairs and reattach it in the basement where the pipe exits to the sewer. You in fact "bypass" the blocked pipe and reroute the water downstream. Sounds easy right? It's slightly more complicated to do this on a human heart but the concept is essentially the same.
Surgeons bypass the blocked coronary arteries with vein or arterial grafts which are hollow tubes like pipes by sewing them to coronary arteries downstream beyond the blockages and get the inflow of blood by attaching them to the aorta upstream. This brings a better flow of blood into the territory of heart muscle that is not being perfused by the blocked coronary arteries. Problem fixed.