The addition operator + can be used as a binary or unary operator.
As a binary operator, it sums two numbers.
As a unary operator, it simply returns the operand without applying any transformation.
Any argument passed as a string to the binary operator is automatically converted into a number.

The following two expressions with a binary operator return the same result.

6 + 4
"6" + "4"

Boolean values are converted into numbers (0 for FALSE and 1 for TRUE) when they are involved in an addition.
Read BLANK handling in DAX for a complete description of the effect of BLANK with different operators.