In a letter of e-mail you might be offered a huge sum of money in return for your help to get money out of a foreign country. Typically, the writer claims to be a senior government official or an accountant.
The writer will tell you that he needs to transfer cash to a bank account in the UK, and that if you let him use your back account you can keep a big slice of the money for yourself.
Once you have given your bank account details your account will be emptied.