Literally, a facade is the front of a building, constructed to make the buiding look larger.
Usually, a facade means a false image that a person projects to mask their real feelings or intent.
"He looked calm, but Sally knew that it was just a facade. Inside, he was terrified that someone would find out about his mistake."
"She acts sophisticated, but it's only a facade. She really is not well-read and knows nothing about art."