“Tongue in cheek” or tongue-in-cheek (there are rules about such things as when to use the hyphenated form but I find them rather unintuitive) is a useful little phrase or figure of speech to express that something you just said, a statement you just made is meant ironically and can’t be taken at face value and should certainly not be taken seriously. It
The expression – obviously – refers to a facial expression – sticking one’s tongue into ones cheek – which IMHO creates a bit of a goofy facial expression and might have originated with people trying to suppress a laugh.
The origin of this phrase is one of these convoluted ones that spans several centuries and continents and so I spare you (and myself) the details.
Instead here is an example:
“I was surprised by Professor Miller’s talk. I always though he was a very serious, humorless guy but many of his remarks were quite tongue in cheek.”