The multiplication operator * multiply two numbers.
Any argument passed as a string is automatically converted into a number.
The following two expressions return the same result.

5 * 4
"5" * "4"

BLANK propagates in a multiplication: if one of the operands is BLANK(), then the result is BLANK().
Boolean values are converted into numbers (0 for FALSE and 1 for TRUE) when they are involved in a multiplication.
Read BLANK handling in DAX for a complete description of the effect of BLANK with different operators.