Though not meant to be taken seriously, it is not overt joking or kidding around, it is "gently poking fun". A "tongue in cheek" statement may have a double meaning, some sort of inuendo or is witty in some way, particularly to the speaker. The tone or the context of the statement may make it to be taken seriously by the listener.
It's origin comes from when Spanish minstrels would perform for various dukes in the 18th century; these dukes would silently chastise the silliness of the minstrel's performances by placing their tongue firmly to the inside of their cheek.
Grandma: I love Dicks, they're so delicious. I think I need seconds.
Kid (tongue in cheek): Hey grandma, why do you like Dicks so much?
Grandma: Oh well I'm not sure, they just make my mouth happy!
Nonetheless beware of jackasses trying to use it as an out-of-jail card sometimes when they fuck up and say some' stupid they really meant.
The following morning: "Hey baby! I was just being "tongue-in-cheek" with that text I sent you yesterday. Just fucking around. Hope you're well. Later.."
person says "the truth is that college educated people are a dime a dozen these days and theirs no real guarantee that you wouldn't be laid off"
receiver of the above statement performs the tongue in cheek action.