A cylinder is like a can of your favorite soda. It has two flat, round tops and a long, smooth side that wraps all the way around. Imagine if you had a piece of paper and you rolled it up into a tube; that would be like the side of a cylinder. The flat tops are like the tops and bottoms of the can, perfectly round and flat.
Think of a stack of pancakes. If you pile them up neatly, they form a shape similar to a cylinder. Each pancake is like a layer of the cylinder, with the round edges lining up perfectly.
Another way to think about it is with a tree trunk. When you look at a tree, the trunk is long and round, stretching from the ground up to the branches. That’s another example of a cylinder shape.
Finally, consider a toilet paper roll. It has a hollow center with a round shape, just like a cylinder. The cardboard tube is a perfect example of how a cylinder can be both solid and open in the middle.