Leto, it's actually a really bad joke, one that makes you groan out loud!

In the joke 'weak' has been misspelt as 'week'. 'Weak' means lacking physical strength or power, whilst 'week' is, erm, you know the calander, 7 days make a week. So the joke is saying that you need a spell check but it is not for the strong and powerful but for those that are lacking strength.
As I said, a bad joke!