Similar to a stereotypical GBF(Gay Best Friend) with the difference being that this friend is straight, A SNTBF won't try to take advantage of you in situations where most other guy friends would, he is generally really sweet and will try to help you to the best of his ability when you're feeling down. When asked, a SNTBF will provide advice or opinions from a guy's perspective. SNTBFs won't be judgmental about you, your actions, etc. SNTBFs are generally also good at keeping secrets, whatever you tell them stays with him and won't go anywhere else.
Girl: What do you think about crop tops?
SNTBF: Honestly, I think girls who wear them are hot! You could totally rock one if you wanted too!
Girl: I just can't believe Jake would do such a thing! I thought I could trust him...I need somebody to vent with right now....
SNTBF: Really? What a douchebag. I get off work in an hour. Want me to stop by then?
Girl: Yes...thanks SNTBF!