The string concatenation operator & concatenates two strings.
With two arguments it works as the CONCATENATE function. However, the operator makes it easier to concatenate multiple strings in the same expression, because the CONCATENATE function only has two arguments and requires multiple calls for three or more arguments.
The following two expressions return the same result.

"hello" & " " & "world"

CONCATENATE (
    CONCATENATE (
        "hello",
        " "
    ),
    "world"
)