"Mean Tweets" is an institution on Jimmy Kimmel Live. Sean Penn, apparently an intentional luddite, told Kimmel that reading mean tweets backstage at the show felt a bit like "taking a selfie." (For those who love a good selfie, he intended it as a derogatory remark.) As someone who just "got rid of email," that didn't appeal to the actor. However, he wasn't outright rejecting the possibility of participating in the segment.
Instead, Penn asked if he could read a bunch of tweets in front of the audience during his interview. Kimmel obliged Penn's whim. He said, by way of an introduction, that Penn hadn't seen any of the tweets before he read them on-air.
Penn was a good sport about the segment, starting it by saying he thought it might be tough for people to come up with anything mean to say, "because [he's] so universally liked." Unsurprisingly, people found a way to say mean things about the Fast Times at Ridgemont High actor. The pile of tweets includes a real gem that starts, "Sean Penn you rude. Dumb rude and full of farts." We can only guess at the punctuation there and assume it was written in all caps.