The quick and simple answer is they will add a little bit more lift to the valve.
If the ratio is 10% more than stock then as an example if a stock rocker opens the valve .300" then the hi lift will open it .330" on the same cam. The point can be argued back and forth as to whether or not any big advantage is gained on an everyday driver. If you need new rockers and like the price I'd get them. If the rockers I have are in good condition then I'd probably reuse them.
