1.
younger brother (humble)
A humble way to refer to one's own younger brother. An older, formal expression.
舎弟がおります。
I have a younger brother.
舎弟は今年大学を卒業しました。
My younger brother graduated from university this year.
舎弟がついてまいります。
My younger brother will accompany us.
2.
underling; follower; junior member
A subordinate or junior member in a hierarchical organization, especially in yakuza or gang contexts. Refers to someone who serves under a senior figure.
舎弟を引き連れる。
To bring along one's followers.
舎弟たちが兄貴の命令に従う。
The underlings follow the boss's orders.
映画の中で、組長の舎弟が裏切るシーンが印象的だった。
The scene in the movie where the boss's underling betrays him was memorable.
Sense 1 is the original meaning: 舎 is a humble prefix (as in 舎兄 for older brother) and 弟 means younger brother. This usage is quite formal and archaic. Sense 2 is the more commonly encountered modern usage, strongly associated with yakuza films and fiction. In this context, 兄貴 (big brother/boss) and 舎弟 (follower/junior) form a hierarchical pair. Related: 弟 (younger brother — standard), 兄貴 (big brother/boss), 子分 (follower, henchman).