Flirting is doing something to attract someone else. For example, you would flirt with your crush. Footsie is an act of flirting (I think of it like a game) where you touch another person's feet and legs with your own under a table secretly.
I don't really know how it affects society, but if done in a relationship, I guess it shows signs that you appreciate your lover. (This is my guess, I've never dated before)
