This all sounds like fairly vulgar language. 'Knob' refers to one particular feature of the male anatomy, so addressing people as 'you knobs' is rather rude way of saying 'you idiots'. .

Some items (including pottery enamel), put a lot of chemicals into the air. Some people breathe in those fumes for the purposes of getting high; "huffing" is a slang term for deliberately sniffing/breathing those toxic chemicals for that reason.

"Knobs" used in this context, is a mild UK-English insult (similar in severity to the US word "jerks") literally translated as "dicks" - as in, penis... It is not exceptionally offensive, and usually used in British English among friends, colleagues or family as banter.