Though Batman is a long-standing member of the Justice League, he at times (many times in fact) does not agree with what they do. Including the "waiting" aspect in terms of supervillains and having them commit a crime before acting.
To counter this, Batman began to assemble a group of young heroes to act as his personal black ops team of sorts. To actively seek out and capture supervillains who he knew were on the loose. As this was outside the scope of the Justice League, and would likely be frowned upon by them, he dubbed them the Outsiders.
The Outsiders have been in many forms over the years, including run by two different Robins in Dick Grayson and Tim Drake, as well as one team being led by Alfred after Batman's "death" in Final Crisis. They've even been shown in Batman: The Brave and the Bold in a more classic incarnation.
Ironically, many of the members of the Outsiders have gone on to be beloved DC Comics characters, including Katana, Black Lightning, and Metamorpho.