"Hang on a sec" ("Hang on a second") and "Hang on a min" ("Hang on a minute") get loads of hits on Google and you will hear English speakers using these expressions frequently - speaking for English English In this case it means, 'wait for a short period of time'.
It's one of few two-word entries - Hang on - in an English dictionary. It means:-
1. To cling tightly to something.
2. To continue persistently; persevere.
3. To keep a telephone connection open.
4. To wait for a short period of time.