You are talking to the program Info, for reading documentation.
There are two ways to use Info: from within Emacs or as a stand-alone reader that you can invoke from a shell using the command info.
Right now you are looking at one Node of Information. A node contains text describing a specific topic at a specific level of detail. This node's topic is “how to use Info”. The mode line says that this is node ‘Help’ in the file info.
The top line of a node is its header. This node's header (look at it now) says that the ‘Next’ node after this one is the node called ‘Help-P’. An advanced Info command lets you go to any node whose name you know. In the stand-alone Info reader program, the header line shows the names of this node and the Info file as well. In Emacs, the header line is displayed with a special typeface, and remains at the top of the window all the time even if you scroll through the node.
Besides a ‘Next’, a node can have a ‘Previous’ link, or an ‘Up’ link, or both. As you can see, this node has all of these links.
Now it is time to move on to the ‘Next’ node, named ‘Help-P’.
>> Type n to move there. Type just one character; do not type the quotes and do not type a <RET> afterward.
‘>>’ in the margin means it is really time to try a command.
>> If you are in Emacs and have a mouse, and if you already practiced typing n to get to the next node, click now with the left mouse button on the ‘Next’ link to do the same “the mouse way”.