When someone successfully submits an entry, you can automatically send them a confirmation email to let them know. You can customize the email to include any follow-up info you'd like, and you can choose to include a copy of their entry in the email as well.
To set up confirmation emails in Form Settings:
You can also create Form Rules to define alternate confirmation emails that will only trigger when certain conditions are met. Form Rules override Form Settings. You can also customize the subject line when you set up confirmation emails through Form Rules.