Q: What is a wormer?
A: A "wormer" is a medication used to help rid your chickens of infestations of internal parasites like round worms or other worms, the same way you might use a wormer for your dogs or cats. However, if you are eating your chickens' eggs---and medications can be transmitted through the eggs---there are only certain types of wormers that should be used, and you may have an "egg discard" period, where you have to throw away any eggs laid (so you're not getting low doses of wormer, too!).
Some wormers do not have a discard or "withdrawal" period because the medication is not transmitted in significant quantities through the egg, while others, like WormGuard Plus don't require you toss your eggs because their ingredients are natural and non-chemical.
You will want to consult a vet before treating your chickens for worms with any sort of medication. Your vet can perform a fecal smear to determine if your chickens have worms and what type they have, in order to provide a good recommendation for treatment. If you treat for round worms when your chickens have another type, your treatment won't do any good!
Another word for "wormer" you may see used is "anthelmintic." Wormers are often called "de-wormers," too!