The + character has three functions in CQL.

First, it is the symbol for addition:

  2+4  6
  # + #  #

Second, (since CQL 6.1) it is the symbol for string concatenation:

  "pin" + "mate"  "pinmate"

Third, it is a symbol for a regular expression special character denoting repetition 1 or more times inside a line filter:

   -- check+

Of course, + denotes "check" inside a PGN file, but this meaning is not used within CQL files themselves.