The Default Email Attachments allows you add an attachment such as Terms & Conditions or another Resource Document to send automatically with your Quote Email.
This can be set by going to your Web Portal > Settings cog > Accounts > Quotes > Default Email Attachments > once you have added the attachment click Save at the bottom of the page.
Attachments are connected through the Resources module within your account.
One off attachments can be added to a quote, find out how here.