The serialization is the process of turning some objects into a data format that can be restored later. For example, you have a forum, online shop or any other web site and you have to send objects between different parts of this site. So, during the serialization you transform an object to a byte stream, so it was in a right form to traverse around HTTP traffic or send to be stored in database.
So, the deserialization is the exact opposite process in which we take structured data from some format and rebuild it to an object.
Most poplar thing today is JSON (JavaScript Object Notation), while recently it was XML, which we discussed in A4.
So, what can go wrong, why is that a problem?