It's a recurring line from the movie Stripes with Bill Murray. Basically means that you shouldn't take some things so seriously. A joke is still just a joke.

>What mean "Lighten up, Francis."?
>
>I see it's not a joke.
>
>>Lighten up, Francis. It's a joke, and a rather good one at that.
>>
>>>I see you as an foreigner xenobhobitist. Don't forget you came there and Indians shared their lands with you. You have to share with others now. We have to share the world...