Concatenation is a fancy word that means "join together"; it is most often used in the computer programming world, which is why you've probably never heard of it.
The ampersand symbol ( & ) is used to join together one string field or string variable to another. The length of the resulting string is the sum of the lengths of the two strings. Numeric data types may be concatenated with strings or other numeric variables or constants. In many cases, the CLIP() function should be used to remove any trailing spaces from a string being concatenated to another string.
Examples: clip(NAM:Lastname) & ', ' & clip(NAM:Firstname) & '$' & ITM:UnitCost