Q: Why was the jelly so stressed out?
A: Because it was spread too thin!

Explanation: Jelly is a food made by boiling fruit juice with sugar until it becomes thick and sort of transparent.  Jelly is similar to jam, marmalade, and preserves.  This link can explain more about the differences between those different spreads.

They are called spreads (noun) because you can use a knife and spread (verb) them on bread (or something else).  Other spreads include hummus and chutney.  If you want a lot of jelly on your bread you can spread it really thick, that is, put a lot of jelly on the bread.  Or you can spread it really thin by just using a little bit.

Spread too thin is an idiom that means that you have too much to do and not enough time to do it all.  If that is you, you might tell someone, “I’m sorry I can’t help you.  I am spread too thin right now.”  Having too much to do can be very stressful.

This joke is funny because it plays with the idea of spread too thin: Literally spread too thin like jelly, or figuratively spread too thin, like when you have too much to do.

If you are spread too thin, you might take a break from it all and learn how to make jelly-