Capacity is how much a container holds. Students compare containers by filling and pouring (which holds more?, which is about the same?). This is a pre-formal introduction; standard units come later.
You already know how to compare things — you can look at two piles of blocks and say which pile has more, or look at two towers and say which is taller. Now we are going to compare something a little trickier: how much fits *inside* containers. This is called capacity.
Think about two cups. One is tall and thin. One is short and wide. Which one holds more juice? It is hard to tell just by looking! The way to find out is to fill one cup with water, then pour it into the other cup. If the water fills the second cup exactly, they hold the same amount. If water spills over the top, the first cup holds more. If there is empty space left, the second cup holds more. This is how we measure capacity — by filling and pouring.
You can also use a scoop or small cup to count how many scoops fill a bigger container. If it takes 5 scoops to fill the red bowl but only 3 scoops to fill the blue bowl, the red bowl has a bigger capacity. You are already good at counting, so this kind of measuring uses a skill you already have.
Capacity is about the *inside* of a container — the space it can hold. Later you will learn special units for measuring capacity, like cups, liters, and gallons. But right now, the most important ideas are: some containers hold more, some hold less, and you can find out by filling and pouring. Those ideas will stay true even when you learn the fancy units later.