Hill Hold Assist (see your owner's manual for a full explanation). You can trigger it by pressing your brake pedal very firmly after coming to a stop. It will hold your brake pedal on your behalf while your car is still in drive (useful for starting on a hill, or for being lazy at a stoplight, etc).
Be careful though: There's been a few stories here about people who've accidentally tapped the accelerator or brake after holding and rolled into another car as a result.