Gas mark is like a special secret code that grown-ups use when they are baking yummy things in the oven. When you bake things, the oven needs to be at a certain temperature so that your treats turn out perfect.
Gas mark is a way to tell the grown-ups what temperature the oven should be, without having to use big numbers or fancy degrees like we learn in school. Instead, gas mark uses numbers from 1 to 9, with larger numbers meaning hotter temperatures.
When a recipe says to bake something at gas mark 4, this means that your grown-up needs to set the oven to be as hot as a number 4 on the special gas mark dial. Then they can put the delicious treat in the oven to bake, and when it's done, it will be perfectly cooked and ready to eat!