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, need a special writing.
Use © to make - Â© - Copyright Symbol
Like I said in the lesson about paragraphs, a browser will recognize a single space, dosen't matter how many you put in a HTML code. But there is a entity which can be introduced to show more then one space.
<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 a entity.
<p> Less then- < <br/> More then- > <br /> Head tag - <head> </p>
Now take a break and practice a bit with this symbols. A bigger list of these you find it HERE. As you will observe in this table, it can be used as well a numerical value replacing the standard name of the symbol.