Click the "Signature Requited checkbox" or select "Only Signature Required" if your are not planning to collect payments thorough your website.
Your Lead will see this widget on their Quote/Proposal: Once your Sales Proposal is signed, you will automatically get notified by email or SMS (Check Action Emails and SMS settings) And your Quote/Proposal will change it's status to Signed, you'll see it in the Leads Timeline and on the Quotes Tab. Also, the PDF-document of your quote will be attached to your lead on the documents tab.