What Is Double Opt-In?
Double opt-in means a sign-up only counts once the person confirms it: after submitting a form, they get an email with a link, and clicking it proves they own the address and meant to sign up.
Single opt-in takes the address as typed. That is fast, but a typo, a fake address or someone signing up a stranger all go straight onto your list, and mailing those addresses hurts your sender reputation.
Double opt-in adds one step: a confirmation email. It costs some sign-ups, since not everyone clicks, but what is left is clean. Bots rarely click confirmation links, so it filters scripted sign-ups too.
It is common, and in some places expected, for newsletters and marketing lists, because it is the easiest proof that someone asked to hear from you. For an ordinary contact form it is usually overkill: people writing to you want an answer and rarely mistype their own address.
The confirmation link should not act on its own when opened. Email security scanners open links automatically, so a well-built confirmation page asks for a click on a button instead.
In WebForms
WebForms has double opt-in as one switch per form on Pro and above. Each submission waits until the sender confirms through the email we send, then it is delivered as usual. Test submissions skip it.