PHP serialize allows you to keep an array or object in a text form. When assigning arrays to things like $_SESSION, it allows PHP to store it in a text file, and then recreate it later. Serialize is used like this for objects and variables. (Just make sure you have declared the class the object uses beforehand)
Wordpress on the other hand uses it for a very similar method, by storing the serialized arrays directly in a database. If you keep a database of keys => values, this could be very beneficial because of the flexibility of arrays, you can store anything in the value parameter.
And heres the link (courtesy of first commentor): http://us3.php.net/serialize
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