meaning of asymmetric on help page for intersect

Sorry if this is a silly question. What does asymmetric refer to from
?intersect, and are there any
aspects of the result that may be different from expected (reason for
exclamation point) ?

As toy data, if needed, here are the examples from the help page.

(x <- c(sort(sample(1:20, 9)),NA))
(y <- c(sort(sample(3:23, 7)),NA))

Thanks,

Juliet