Optimized publishing means that only content in the project that has been scheduled will be transmitted to the Player(s) when publishing.
You can choose the optimized publishing option when setting up a Player connection. If you add a Player through Bonjour, you'll have to double-click on it in the list in order to enable optimized publishing.
Calendar schedules, timer playlists and anything set to be remotely startable will always be transmitted, regardless of whether optimized publishing is enabled or not.
The advantage of optimized publishing is that it saves hard drive space on your Player by only publishing the contents in the project that are relevant to that Player.
Transfer speeds for projects may vary according to connection type.