The logical or operator || returns TRUE if any of the arguments are TRUE, and returns FALSE if all arguments are FALSE.
With two arguments it works as the OR function. However, the operator makes it easier to include multiple conditions in the same expression, because the OR function only has two arguments and requires multiple calls for three or more arguments.

The following two expressions return the same result.

[measure A] > 10 || [measure B] > 20 || [measure C] = 30

OR (
    [measure A] > 10,
    OR ( 
        [measure B] > 20,
        [measure C] = 30
    )
)