What is meant by freeze in League of Legends?

Freeze in League of Legends refers to keeping the minion waves at a set location. The minion waves get stuck neither shifting to your tower or the enemy's tower. As a rule of thumb to successfully freeze a lane, you want more enemy minions than ally minions. It is impossible to keep a lane frozen, but by lasting hit minions and attacking them more only to prevent the minions from going under your turret, you help freeze the lane.

Example: Freeze the lane Ashe so Vayne misses minions