What is diversion program, and will this 'seal' my criminal record after completion?

It means that if you accept the diversion program instead of fighting the charges in court you will not have these charges on your record if you do not get in trouble during the time the program is in effect, but you will still have to get the arrest records expunged after you complete the program. Although you will not have a conviction record, your arrest record will still be on file unless you apply to have them expunged.