A wave indicates when a migration will happen. You can define as many waves as you like using the Add Wave screen. Waves can be seen on the Planner Tool schedule. Waves normally have a name that is based on a quarter but you can create them with whatever label you like. You can see one here labelled Horizon.
The Planner Tool schedule will shows the waves in date order left to right from earliest to latest. If you change the date of a wave it will be re-ordered in this grid view when you next visit it. Changing the date will have an impact on the model when you next click the Implement button in the planner tool. Any application or infrastructure associated with that migration wave will be moved to aling with the start date for the wave.