He's a Youtuber who's held the top spot (in terms of number of subscribers) for a long time now. He started out doing gaming videos called 'lets plays' and then moved over to being more of an internet comedy guy. He's gotten a lot of heat recently as the legacy media made various attempts to come after him for 'Nazi posts', misogyny, saying the word nigger on a livestream and whatnot - though he's pretty apolitical and is for the most part just a comedy channel. The negative attention cost him some contracts with disney and a Youtube Red show and Youtube itself (probably) has/had him listed in their secret backend ratings system as adult-only content, blocking most ad revenue on his videos in recent years. Predictably, the unfair negative characterizations, though they cost him a lot of money, actually made him more popular as sort of a counter-PC-culture icon. His content is like black licorice - most people either love it or hate it. If you want a really good explanation on the recent battle vs t series: https://quillette.com/2018/12/20/pewdiepies-battle-for-the-soul-of-the-internet/