You have some email addresses of potential customers that you found on the internet. They are called 'cold' contacts because you've never had any conversations or communication with them before. Brrrrr.
A few fine chaps have signed up for your newsletter on your website. Those clever people have now 'opted-in' and have given permission for you to email them as much as you like (remember they'll take it back if you annoy them).
Here's the clever bit. If those subscribers then click on a link you have sent them to confirm they've subscribed, they are then 'double opted-in' and you can be sure they want to hear lots from you.
The big conundrum for us marketing folk is whether that email you send them to confirm actually annoys them. That, we don't want. We think it can be better to encourage some sort of action, like clicking a link through to your website, so we can see they are interested. A softer approach. It does, of course, depend on the industry and business. Everyone is different.