If you are allergic to a thing, it is best not to put that thing in your mouth, particularly if the thing is cats.

This quote is saying that if you have an allergy to something, like cats, you should avoid putting that thing into your mouth. This is true because doing so could cause an adverse reaction or even make your allergy worse. It is important to take precautions when dealing with things that you are allergic to in order to avoid having a dangerous reaction.