Expungement is a legal process, under Penal Code §1203.4, where someone can seek to have a previous felony or misdemeanor conviction dismissed.
You are eligible to receive a dismissal if you were granted and completed probation and are not currently on probation or serving any other sentence.
You can also seek a dismissal where you were denied probation, or your case was reduced to an infraction (§1203.4(a)).
If your dismissal is granted, a new entry is made to your criminal record showing the case was dismissed. You can answer on many job applications, but not all, that you have no convictions. However, when applying for a government job, a job that requires security clearance, or a job that requires a government issued license, certificate, or permit, you should disclose the conviction along with the later dismissal (expungement).
You are not eligible for an expungement if you were sentenced to state prison.
A dismissal of your case DOES NOT ERASE OR REMOVE YOUR PAST CONVICTION. It is an official acknowledgement that although you had a conviction, the court is satisfied that you successfully completed all the terms and conditions of your probation, and for that reason, the court grants a dismissal.