In the UK, culturally we have a small thing to eat mid morning. Adults might just have a drink (usually hot), but typically it will constitute a drink and a small food item such as a biscuit, piece of fruit, bit of cake or a small sandwich, piece of cheese.
You may or may not have similar mid afternoon and then at some point before you go to bed. It's normal to have these built into your routine. It won't cause comment if you don't bother, for whatever reason.
Very small children often have this baked into their routine - again, this is cultural and not mandatory. If you're going out somewhere then it's much cheaper and more convenient to bring a small snack item from home than get fleeced in Costa for a flapjack.
When your child goes to school you'll be expected to provide a morning snack. No-one will call social services if you don't do this and your child doesn't care to have it. The school may have a scheme where they provide a piece of fruit in any case.
Just because a particular culture does one thing doesn't mean you have to do the same, but that's what a snack is. Just in case you really didn't "understand".