Usually a 5-sequence letter is an email used in direct marketing, sent via autoresponder, which walks customers through a sales sequence.
This form of email marketing is a way to set up your email autoresponder so that it automatically sends a message to your list members when they take certain actions.
They'll automatically receive the message via your autoresponder and you don't have to initiate it.
Sending triggered email messages is also a common feature of email marketing autoresponder services.
If you're in the market for an email autoresponder service, you should make sure that it lets you segment your list for email marketing messages so that you can employ this method.