A histogram is like a graph that shows how many of something you have. Let's say you have some toys, like cars and dolls. You put all the cars in a box and all the dolls in another box. Then you count how many of each toy there are.
Now you can draw a graph to show how many cars and dolls you have. The X-axis is like a line that goes left to right, and it shows the different numbers of toys you have (like 0, 1, 2, etc.). The Y-axis is like a line that goes up and down, and it shows how many toys you have of each kind (like 0, 1, 2, 3, etc.).
So you draw some rectangles on the graph, one for the cars and one for the dolls. The height of each rectangle shows how many of that toy you have, and the width of each rectangle shows the range of numbers (like maybe you have 0-5 toys, or 6-10 toys, or 11-15 toys).
When you're done, you can look at the graph and see which toy you have more of. Maybe you have 8 cars and only 4 dolls, so the car rectangle is taller than the doll rectangle. The histogram helps you understand how much of different things you have, and compare them to each other.