BIN stands for “Buy It Now”.
It’s a price of a domain that you can pay to purchase the domain immediately instead of having to spend time bidding on the domain in the auction.
The whole point in a BIN is if someone REALLY wants your domain they can pay the BIN price and bypass all the work of having to participate in the auction. However, the price of the BIN is usually pretty high in comparison to the auction’s starting price.
So at the end of the day it really comes down to how badly you want the domain and what way you think you’ll be able to get it cheaper – if getting the domain is your #1 priority then paying the BIN price might be better but if you’re just looking for a good deal you might want to spend the time participating in the auction.
Hope that helps.