I Give Up.. What are Collector's Bills?

I searched this subreddit 3 or 4 pages deep and found a lot of chat about them, but couldn't find anything that told me exactly what there and how you use them?

I'm taking it that happenstance will periodically award you some when doing a web search using their engine.

But what does one do with 'em?

New here and searching on my own didn't provide an adequate answer.

tia for any and all help....