Parameter syntax:

[ 'before' ]?

Insert a new block, similar to the current one, after the current one. If option before is specified, the new block is inserted before the current one.

To make it simple, a block is a paragraph or a list item. However what exactly is a block must be specified as explained in Section 50.1, “Specifying splittable blocks”.

This command can be executed if there is no selection or if a single element is explicitly selected. When there is no selection, the current block is automatically determined among the elements which directly or indirectly contain the caret. When an element is selected, this element is considered to be the current block if and only if it directly or indirectly contains the caret and it is listed in the configuration_name blockList property.

This command is intended to be bound to keys Ctrl+Enter and Ctrl+Shift+Enter.