What does it mean if your supervisor recently fired an employee for not being punctual?

It normally means the employee was terminated as a result of not being on time to work, or late. Normally, most employers will give an employee a warning prior to terminating the employee, and the behavior, such as tardiness or punctuality, has occurred more than once or twice, normally it is several times, and at least one warning. In California, and most other states, Labor codes allow for "at will employment", and unless there is an employment contract in place, almost all employers and employees are subject to "at will employment", the employer can legally terminate an employee, with out cause.