What is the groovy

In groovy, the bitwise operators can be overridden with the leftShift (<<) and rightShift (>>) methods defined on the class. It’s idiomatic groovy to use the leftShift method for append actions on strings, buffers, streams, arrays, etc and thats what you’re seeing here.

For example:

• The overloaded leftShift methods on OutputStream which are used to append bytes, an InputStream, or an Object to the stream.
• List, which also uses it as an append

You are looking at a grails tag lib, so out represents the page that’s being rendered. The results of this taglib will be added to the output buffer that will be rendered to the client.