An assistant coach in basketball is a coach that is apart of the coaching staff for a team, but is not the head coach. They can specialize in a certain area of basketball such as offense, defense or shooting. A team can have many assistants on staff that focuses on any area of the game.
Recruiting is a huge aspect of an assistant coach's job description. It is impossible for the head coach to be in charge of looking at every high school or college player that may be a fit for their team, which is why the burden mainly falls on the assistants. The difficulty in recruiting is not just the scouting of talent that will mesh well with the roster, but also making the players want to join the team. Especially in college basketball, assistant coaches have to be able to differentiate their programs from other schools.
Some of the larger scandals in NCAA history have come from assistant coaches doing too much to try and recruit players such as giving them monetary benefits.
In terms of actual game preparation, assistants are tasked with coaching practices and refining a player's skill set just like the head coach is. Whether it be working on shooting form or footwork, assistants are experts in the field of basketball.
During games, assistants may be looking at stats or fouls, making sure to relay information to the head coach. The head assistant is the coach who will take over if the head coach is ejected from the game.