Identity theft is where criminals obtain personal information such as your date of birth, bank account numbers, your mother's maiden name, your pet's name and so on.
These personal details are often used as questions by companies for you to prove who you are over the telephone. So, if the thief can answer these questions there is a good chance they will succeed in impersonating you.
This is bad news, as they could purchase things in your name, take out loans, even take money from your accounts!