Concatenation refers to a series of things — ideas, events, animals — that are somehow interconnected, individual parts that are linked to form a single unit, like the links in a chain.
If you think about a chain, you can imagine the individual links — they move separately, yet are linked so they always move together as well. This aptly describes concatenation, the state of several things being bound together. We can see the meaning from the word's source, the Latin concatēnāre. It comes from catēnāre, "to make a chain, to link," which itself comes from catena, "a chain." Add the prefix con-, meaning "together," and we get the meaning "to link together."