The + operator is used for string concatenation in JavaScript. When one of the operands is a string, + performs concatenation instead of addition. For example, 'Hello ' + 'World' results in 'Hello World'. While the concat() method can also be used (as in 'Hello '.concat('World')), the + operator is more commonly used for string concatenation due to its simplicity and readability. Note that & is the bitwise AND operator and || is the logical OR operator in JavaScript.