"bestehen" = to be made of, to exist, to pass (a test)
These are the three major meanings of "bestehen". The important thing in this sentence that indicates the "made of"- meaning is the "aus". Other ways to say this are:
"Sie sind aus Plastik", "Sie wurden/sind aus Plastik hergestellt/gemacht/gefertigt/erbaut/zusammengesetzt."
If you have something like "aus /stuff/" it normally indicates that it is made of this /stuff/. The most often used verbs in this case are "bestehen" and "sein".
I hope I could be of help.
i checked some dictionaries and bestehen was translated to consist, exist, insist, pass, so i wrote The chairs consist of plastic. It does sound odd in english to me, but is it wrong?