Pages in Plone vary greatly, but are single "web pages," of one sort or another.
To add a page, use the Add new... menu for a folder:
Select Page from the drop-down menu, and you'll see the Add Page panel:
The Title and Description fields are there at the top. Fill each of them out appropriately. There is a Change note field at the bottom, also a standard input that is very useful for storing helpful memos describing changes to a document as you make them. This is useful for pages on which you may be collaborating with others.
The middle panel, Body Text, is where the action is for pages. The software used for making Pages in Plone, generically called visual editor and specifically a tool called TinyMCE, is a most important feature allowing you to do WYSIWYG editing. WYSIWYG editing -- What You See Is What You Get -- describes how word processing software works. When you make a change, such as setting a word to bold, you see the bold text immediately.