Meaning of “plain” - Learner’s Dictionary

plain

adjective us uk /pleɪn/
Extra Examples
She was dressed in very plain clothes.The room was furnished with a plain table and chairs.This plant has very plain flowers.The walls are made of plain brick or stone.The decor was plain and simple.
SIMPLE

B1 simple and not complicated:

plain food
NOT MIXED

B1 not mixed with other colours, substances, etc:

a plain blue carpet
plain yoghurt
PERSON

A plain person is not attractive to look at.

OBVIOUS

obvious and clear:

[ + (that) ] It's quite plain that she doesn't want to talk to me about it.
→ See also be plain sailing

(Definition of “plain adjective” from the Cambridge Learner’s Dictionary © Cambridge University Press)