In HTML, "entity" is a fancy name for "symbol". Some symbols as copyright, trademark, and many others, some of them are available on your keyboard, but others need special writing.
Use © to make - © - Copyright Symbol
As I said in the lesson about paragraphs, a browser will recognize a single space, doesn't matter how many you put in an HTML code. But there is an entity that can be introduced to show more than one space.
html<p>In this phrase there had been introduced more spaces.<p>
To make commands in HTML you use the "less than" and "greater than" symbols. For being showed in the browser you should use an entity.
html<p>
Less then- < <br/>
More then- > <br />
Head tag - <head>
</p>
Now take a break and practice a bit with these symbols. A bigger list of these you find it HERE. As you will observe in this table, it can be used as well as a numerical value replacing the standard name of the symbol.