Martial arts hand symbols
Usually our first experience with martial arts hand symbolism probably came when we saw our first epically bad kung fu movie with the dubbed audio and the lips that don't match the words being said and then from there we probably watched a horrible ninja movie or a JCVD (Jean Claude Van Damme) movie where you see the main actor sitting in an uncomfortable position and throwing up hand signs while they are meditating (what a load of bollocks btw).The first one is that it is a salute or greeting to a stranger to identify which gang/tribe you support or which army that you belong (a lot like a secret hand shake). If you think critically about this thought or idea though it becomes apparent how dumb this is. Its dumb in the sense that anyone can throw up a hand symbol and communicate that they are allies and then only turn out to be your enemy. Being a spy would have been way to easy if your only form of background check was putting your hands together in an elaborate symbol. People must have been dying left right and center.
As for hand symbols while meditating, I'm sure it has meaning I just don't know anything about that yet. Perhaps its a movie directors attempt at making meditation more interesting.