Inside Macintosh: AppleScript Language Guide / Part 2 - AppleScript Language Reference

Chapter 3 - Values / Using Value Class Definitions

Operators

You use operators, such as the addition operator (+), the concatenation operator (&), and the equality operator (=), to manipulate values. Values
that belong to the same class can be manipulated by the same operators.
The "Operators" section of a value class definition lists the operators that
can be used with values of a particular class.

For complete descriptions of operators and how to use them in expressions,
see "Operations," which begins on page 161.