The look on your face — sad, happy, bored, amused — is an expression. Your expression lets people know what you're feeling, unless you're good at hiding your emotions.
Expression comes from a Latin word meaning “to press out” and it can still be used this way. If you make fresh orange juice, your juicer helps with the expression of the juice from the fruit. If someone uses art to express their feelings about the world, that's artistic self-expression: something felt inside has been brought out and shared. We also use expression for ways of saying something, like the expression "what's up?"