He's referring to the PCV system (positive crankcase ventilation?). I had to do the same thing on the 327 in my '67 Impala.
Google PCV for the specifics on function, purpose, setup, pro's, con's etc.. That'll help more than I ever could.
For my 327, I had the stock valve covers (no breather holes), with the oil cap/breather being a tube at the front of the engine, and the PCV system was a valve in the rear of the engine (near the dizzy), with a hose running from there into the front of the carby.
The new valve covers have a hole in each for the breathers. Here's what you need to do:
Remove the oil filler tube at the front of the intake, and plug it with a freeze plug. The valve cover with the hole for the oil cap (you'll need a new oil cap if not supplied with the valve covers) now acts as the oil filler/breather.
For the breather on the other valve cover, you need to insert the grommet and valve (should be supplied), and run the hose from there to the vac port on the front of your carby.
Then you need to remove the old valve from the rear of the engine, and plug that hole with a freeze plug.
Hope that wasn't too confusing.
I think this is how they did it from 68/69 onwards, which is why pretty much all aftermarket SBC valve covers have breather holes.
Here's a pic of mine before the switch. Note the oil filler tube at the front. And the thick hose running along the intake from the rear to the front of the carby? That's the PCV hose.
Here's after. Note the plug in the front where the old oil filler tube was (the same was done to the rear where the pcv valve was), and the breather valve and hose coming from the valve cover to the vac port on the carby. The other valve cover has the oil cap/breather.
Hope that helps.
Good luck!
