Essentially, an overdraft is when a bank allows you to spend more money that you have in your account, up to an agreed amount. Providing that you do not exceed the overdraft then you will not be charged for using your overdraft.
If your bank allows, you can increase the maximum overdraft limit. Some banks will usually consider this at the end of each academic year. Some will expect you to apply for an increase, some will offer it automatically. Much will depend upon how you have managed your overdraft limit, i.e. stayed within the current overdraft limit.