A pharmacopoeia is a book that contains a list of all the different medicines that can be used to treat illnesses or help people feel better. It's like a giant menu of all the things you can take to make yourself healthy or feel better.
Every medicine listed in a pharmacopoeia has to be approved by doctors and scientists first, to make sure it's safe and works the way it's supposed to. That means you can trust the medicines in the pharmacopoeia to do what they say they will do and not hurt you.
The pharmacopoeia is like a big dictionary, except instead of words and their meanings, it's full of medicines and their instructions for how to use them. Doctors and pharmacists use the pharmacopoeia to find out what medicines are good to prescribe for different illnesses or symptoms.
In short, the pharmacopoeia is like a big book of safe and approved medicines, that doctors and pharmacists use to help you feel better when you're sick or have pain.