A go-to bag, for example, would be one that went with any day-to-day outfit and is suitable in pretty much any situation. If you're in a rush, you grab your go-to bag.
If you're feeling unimaginative, you eat your go-to food - something quick that requires little effort but tastes good.
The same principle can be applied to pretty much anything.
B: Nothing, this is just my go-to face.
A: Dude, all you have in your cupboard is pasta and tinned tomatoes.
B: Yeh I know, I haven't had chance to go shopping - pasta is just my go-to meal.
A: Do you know what kind of glue I should use to stick 2 pieces of wood together?
B: I dunno, I'd just use PVA, it's my go-to glue.
The term perfectly describes a relationship based soley on sex. Unlike "friends with benefits" which implies friendship that includes sex, the "go to" relationship generally starts as sex first. If a friendship develops it is usually a by-product of the sexual relationship rather than the origination of the sexual relationship. Different than a booty call or hook up, the go-to relationship is generally more consistent, which eliminates one or both parties from having to go out and look for it.... Cuz ya got a "go-to" for that!
Question: What are you up to? Answer: My go-to is coming over for a bit then I'm going to the mall.
Question: Wow, he's cute...are you dating him? Answer: No, he's my go-to.