This can refer to any item or object which is the most commonly used or which you use without having to think about it, such as a food, bag, facial expression, etc. Your 'go-to x' is the 'x' that you will resort to most commonly when you don't have the inclination/time to come up with something more original, or else that you will just go to automatically.
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.
A: You look grumpy, what's up?
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.