The term bet can be used in a few different ways, but generally means "agreed" or "okay". For example, if someone says "Wanna grab lunch later?" and the other person responds "Bet", it means they agree to the plan. Bet" can also be used as a response to confirm that a statement is true.
The BET 500 was light car model developed by BET, a small Greek vehicle manufacturer. Introduced in 1973, it used a Fiat 500cc engine, had a metal body and seated up to five passengers.