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

The following two expressions return the same result.

[measure B] = 30 && [measure A] > 10 && [measure A] < 20

AND (
    [measure B] = 30,
    AND ( 
        [measure A] > 10,
        [measure A] < 20
    )
)