What is this icon that looks like a headphone cable? My phone got unlocked in my pocket and appears to have changed some things in the Google Now app (although I'm not sure this icon is related to that). I have never seen this icon anywhere before. I am using LG G4 stock android 6.0.

What is this icon and how can I revert whatever change caused it to be displayed?
The icon looks like an aux cable which suggests that your phone thinks that you have plugged in some headphones.

Assuming you haven't, you can try peering into the headphone socket (use a bright light) to see if anything is trapped in there. If there is you can try blowing into the port to see if that will dislodge/remove it, or turn if so the port is at the bottom and tap gently to see if that helps.