Not all characters possible for rendering on your web page can be entered into the HTML code. For example you have reserved character such as the angle brackets < and > that are a part of HTML tags. You also have characters that you cannot type on you keyboard such as the copyright © character.
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These characters are entered into the HTML content as entities. Entities begin with an ampersand & and end with a semicolon ;. Between them is either an entity number or and entity name. Entity numbers are preceded by a hash symbol #. For example the registered sign ® entity number is 174 and you use it as ®. The entity name version for the registered sign ® is ®. The non breaking space prevents the line break for spaces between other non white space characters. White space characters are tab, return and space bar.
A common entity is the non breaking space. The entity number version is  . The name version is
The following are reference pages for looking up entity names and codes.
Entity Code. Commonly used HTML symbol entities codes grouped into categories with character rendered, entity name, entity number and description in a table.
W3C Development. This is a supporting page from their HTML5 reference documentation page. You need to roll the mouse over the table to see the entity codes.