Generally, the Nigerian 419 scammer claims to be the relative of some high-profile deceased/arrested political figure. The scam claims this relative has horded or saved millions of dollars in cash and are now seeking someone to help transfer the money in exchange for a hefty percentage of the profits.
Sound to good to be true? It is. Those who have contacted the scammers are presented with many forged or official documents in an attempt to convince the victims that the promise of money is legitimate. Meanwhile, they claim fees and bribes must be paid and accounts must be setup. All this money is at the victim's (your) expense and there is no payout at the end.
But loss of your own funds may only be the tip of the iceberg. Some who have responded to Nigerian 419 scams have become victims of kidnapping and murder. So beware and remember, there's no pot of gold at the end of rainbows and an email promising you millions of dollars is a scam.