Originally a sticker on the back of cars used to denote a surfer, bodyboarder, or general beach bum whose life centered around the ocean or related beach going activities. Ever seen a 32 year old man whose primary activity was surfing and working on his tan? That person would have a salt life sticker.
Unfortunately the trend spread to senior citizens, wanna be hipsters, soccer moms, and other persons wishing to inform the world that yes, they too have visited the beach at some point in life and they have the sticker to prove it.
Dude: "Check out this wicked Salt Life sticker I just got for the back of my car!"
Me: "That's really lame dude. My Grandma has one of those on the back of her wheelchair and aren't you afraid of the ocean ever since you got raped by that dolphin?"
Dude: "I don't want to talk about it."