If it starts with a "W:" it's not an error, it's just a warning. What this means is that the key signature for that repository is invalid/expired - which in turn means that the repository still works fine, it's just the "key" used to authenticate it is no longer formally trust-worthy.

In THEORY, this could suggest a man-in-the-middle attack of some sort, but most likely, it's just that - an expired key. No biggie.