If you develop already for Android, then you know what it is so you can skip to the next section.
For the others of us: a toast is a spécial way to display 'non intrusive' message to the user. Those message are displayed on a configurable place on the screen and they disapear after a configurable time interval. The way they appear is similar to the way the Growl app (on mac do).
A toast is a view containing a quick little message for the user. The toast class helps you create and show those.
When the view is shown to the user, appears as a floating view over the application. It will never receive focus. The user will probably be in the middle of typing something else. The idea is to be as unobtrusive as possible, while still showing the user the information you want them to see. Two examples are the volume control, and the brief message saying that your settings have been saved.