Jobs are a collection of privileges and settings, that you can use to make sure the employee has the access they require in order to do that Job.
For example, bar staff may not need access to tables but servers would. They may also have different pay rates. It is possible that the same member of staff would be waiting on tables one day and tending bar the next. Instead of giving them access to everything they need for both jobs you assign them the jobs of server and Bar. When they log in they can choose which job they are doing that day and they will be assigned the correct privileges and paid the correct amount of money.
You can set it so that you have to be clocked in and assigned to a Job before you are allowed to log on to the system