Play Lissandra and ult people to freeze them (just kidding). Freezing a lane is when the opponent's creeps are pushing towards you but you do not let them move forward. You stand there at the bridge telling them they cannot pass, and they just hit you for a while as you wait for your own minions. Freezing lanes is an important skill for controlling your lane. Freezing allows you to deny your opponent creep kills, and also forces your opponent overextend to farm and get experience.