It is a way of encouraging them to lay eggs in the nest boxes. Chickens, when they are young and just starting laying eggs, will lay them anywhere. The thought is that if they have something hard and round under them, they will start putting their own hard and round objects in the same spot.
I tried it, and my hens scratched a hole in the coop and practiced their putting (just kidding!).

It's when they start practicing their drives that it's time to worry! *G*

Yeah, the golf ball trick is an old one...works sometimes, not others. Although sometimes my silly girls will auddenly get it into their heads that weird places are the PERFECT place to lay an egg! Especially the banties, the little stinkers...

Enjoy your chickens!

Velvet ~:>

You might find plastic toy eggs in a toy store.

Some people recommend wooden eggs.

I used plastic toy eggs years ago. Worked just fine.

Amazon has some wooden ones for $12 a dozen. They list 877 items under toy eggs. plenty to look at.

Here is a link that might be useful: toy eggs at Amazon including wooden