Slitherine

On the multiplayer screen "Accept Challenge" what's the difference between white and red padlocks.

If a challenge is called a "Ranked Challenge" what does this mean?

TIA

White padlocks are open games that anyone can join. Red padlocks are private games that require a password.

Ranked Challenges affect your ELO rating, which was recently added to the game. Unranked challenges will not affect your ELO.