Open Question: what does it mean when you plea first offender?

i plead it 7 years ago and it was a mis. for disorderly conduct. also when i got arrested for theft by recieving i plead nolo and got no consiquinces was it dropped? got a background check mon and worried but i have had them before military and walmart so idk just nervouse bc the application said state them unless its been sealed or expunged and i really thought it was bc it was first offenders and the other was dropped i thought but military found record when they called judge and old probate cause i felt guilty saying no and they had to search. anybody got advice?