The expression '10' + 5 will evaluate to the string '105'. When the + operator is used with a string and a number, JavaScript performs string concatenation rather than addition. The number 5 is implicitly converted to a string, and then concatenated with the string '10', resulting in '105'. This is an example of type coercion in JavaScript, where the numeric operand is automatically converted to match the string operand when using the + operator.