A character entity is a method used to display special characters normally reserved for use in HTML. For example, the less than (<) and greater than (>) are used as part the HTML tag structure, so both symbols are reserved for that use. But, if you need to display those symbols on your site, you can use character entities. For example:

use < for the less than (<) symbol
use > for the greater than (>) symbol

Reference http://codex.wordpress.org/Glossary