They come in blue and orange and are made by Clover. You pess in on the side bar and it releases from the hook so you can slip it through a stitch.
I use them to keep track of the last and first stitch in each row and to hold my current stitch when taking a break.
I have the smaller size, too, but if you're using chunky yarn or you're following an Aran crochet pattern and have lots of thick cables it's easy to lose sight of the small ones.
It's easier to locate the bigger markers and it's also easier to pick them up.
I usually have mine sitting on the end table next to me while I work and when I go to pick them up I can't get a grip on the small ones whereas the thickness of this size allows you to get a grip.
My mother-in-law had been working on a ripple blanket and constantly having to undo rows when she lost track of where to put the next stitch. I gave her a couple of my markers and she was shocked at how much they helped. I was surprised she hadn't used one before because she's been crocheting as long as I have.