What are drop targets in gedit's snippets?

There is a plugin for gedit to handle snippets. In the Snippets Manager under "Activation" for every snippet you can set the drop targets. There are some preformatted alternatives to choose between but you seem to able to compose your own alternatives too. The preformatted alternatives are: text, text/plain开发者_JS百科, text/xml, image, image/png, image/jpeg, audio and video.

What are drop targets? How do they work?

Below is a screenshot, the "Drop targets" field is visible under "Activation" in the lower right: