The mud is called Lena Blackburne Baseball Rubbing mud and allows pitchers to have a better grip on balls. Before this mud, baseballs were rubbed in water and dirt and sometimes tobacco juice and shoe rub. https://www.mlb.com/news/five-things-you-probably-didnt-know-about-baseball/c-45656360

For decades, all the balls used in Major League Baseball have been coated with mud harvested from a secret spot along a Delaware River tributary in New Jersey. Only a few people knew of the exact location, and until recently, only a few knew the exact contents of that mud. https://www.nytimes.com/2024/11/04/science/baseball-mud-mlb.html