Student Reps exist to represent students on their course or department, and they raise problems (as well as successes), with the Union and the University which we then fight to resolve! Reps bring issues about their course or student experience (halls, buses, catering etc.) to University staff to develop and enhance their experience.
You can give as much time as you can. You are here to study afterall. You will be invited to meetings with the University or invited to take part in focus groups. The more you do, the more you will be recognised by the Union and University. It is all about giving back to you course and your fellow students, so they will be looking to you for feedback. The role usually takes 1-2 hours of your time per week, so shouldn't take too much of your time.
You will receive training on a series of topics, inlcuding how to communicate effectively and dealing with student issues. You learn transferrable skills that you'll be able to use throughout your uni career, and beyond.