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This is a library of custom widgets created by WEM and by partners of WEM. This library grows continuously and has a lot of different widgets. When you drag the rocket to your template you get prompted to select the widget you want to add.
The first time you do this there are probably not many widgets listed, and you need add a library first. Select the 'Add more widgets here' link and select a library you want to add to your account. Take a look around here. There are a lot of different widgets and every one is for a different use-case.