If it's a Multi-port valve, then it has a "spider gasket". With the power shut off to the pump, remove the top of the valve (handle, etc), inside is a gasket that is shaped a little like a spider's web, hence the name.
These are pretty easy to replace as long as you do an awesome job of cleaning out the old one. Lube and properly seat the new one and re-assemble the top of the valve and you're done.
To "tap" the line basically means to install a valve that you can close all the time on the backwash line. Just remember to open it before you try to backwash
If you have a "Push-pull" valve, it's a very easy fix: unscrew the screws on the top, pull the piston out, replace all the O-rings, lube them and re-assemble. Very basic.
Another trick, that sometimes works when the multi-port valve leaks, is to sipping the valve all the way around, past filter, and back again. This can help ensure proper seating.