Concatenation (from Latin concatenare, to link together) is taking two or more separately located things and placing them side-by-side next to each other so that they can now be treated as one thing. In computer programming and data processing, two or more character strings are sometimes concatenated for the purpose of saving space or so that they can be addressed as a single item. (Of course, some way may be needed to know how to unconcatenate them later.)
A simple way to remember concatenation is to think of any word. What makes a word separate from other words is that all the letters are lined up together with no intervening spaces. That is, in a word, all letters are concatenated. Sometimes new words can be made by concatenating two existing words - for example, "airline" is a concatenation of the words "air" and "line" into a new word.