An 'Estimate' can be an offer that you make to a client -or a prospective client. It could be a simple sales quote. Any business proposal, really, for the purchase or a service.
You can use the "Estimate" functionality to send your clients those offers, allowing them to accept estimates directly through Eloru's client portal. If you wish to simply use their email, you have the tools to manually mark acceptance (or rejection), as well.
Most importantly, you can use the functionality to create invoices directly. Product lines among estimates and invoices are identical. Within a couple of clicks, client details and item lines can become part of an invoice . ready to be sent to the client who has accepted his office.