Corking reduces weight, so a batter could use a bat with a bigger barrel, which makes making contact easier, but the reduced mass reduces power. It’s become a lot less common.
When Sosa went up against McGwire for the home run crown he is may have used a corked bat, in addition to steroids. He was caught using one in 2003, although he claimed it was an accident and he only used the bat for batting practice. While it’s possible he didn’t use a corked bat in ’98, and he has had dozens of bats checked and all came back clean, it is an interesting question. If he did use a corked bat, and it actually hurt rather than helped, could he have hit more than 66 home runs?