This is a crowd of bacteria that is created by reproduction. A colony contains many bacteria of the same species and differs in size, structure and colour from colonies of other species. Sometimes bacteria can be identified by the appearance of their colonies.
Explanation for children:
Bacteria do not like to be alone, they like to have their conspecifics around them. A bacterial colony is a collection of bacteria of the same species. You can think of them as a school class. The students from class 1a like to be together, and the students from class 1b as well. If you compare their class photos, the two classes look completely different. You can also compare bacterial colonies, because different colonies have different structures and colors. Some of them look like small volcanoes, others like white spots, others like red spots, and so on. And because there are lots of bacteria in a bacterial colony, you can even see them.