Color Catcher sheets are very much like fabric softener dryer sheets, only instead of putting them in the dryer, you put them into the washing machine. They're for catching any color that might bleed out of whatever you're washing. (You can find them in the laundry products aisle of your store.)
I had occasion to try them for the first time recently. I was washing a couple new fabrics before sewing them, and I wanted to make sure they wouldn't bleed onto each other. I used two Color Catcher sheets, just to make sure, since the first step in the directions said to use two for new garments.
They did the job, and here are a couple of quickly-taken-in-poor-light photos to show you how they turned out. The center sheet is a new, unused one, and on either side is a used sheet.
You can certainly tell the difference. There was a little bit of bleeding, although it was probably not enough that it would've affected either fabric.
It's nice to know that I don't have to worry about dye bleeds anymore. I can think of numerous occasions in the past when I held my breath washing something new in with other clothing!
(No, this was not an ad and I was not compensated in any way. It was just my own personal experience.)