A quote is a non-binding price estimate for a customer. It shows what goods or services will cost, without a final agreement having been made, and without affecting inventory or accounts.
What it is used for
Give the customer a price before anything is agreed.
Gather several lines (goods/services) into one combined estimate.
From quote to sale
If the customer says yes, the quote can be turned into a sales order, so you do not have to type it all again. The sales order can then be invoiced, and only then are inventory and accounts affected.
Read more in Quotes.