When you're introducing someone over email, you want to make sure all parties involved are completely comfortable with the intro. The "double opt-in" approach gives both parties a chance to decline without causing any problems.
Picture: Tim Snell
A "single opt-in" is when you send an introduction email to someone without clearing it with them beforehand, and it puts everyone in a tough spot. If they decline the intro, the person hoping to get introduced will see it since they're likely to be copied in to the conversation. Additionally, the person you sent the introduction email to may not have any desire to connect with who you're introducing in the first place; wasting everyone's time. That's why Anand Sanwal at Quartz recommends you use the "double opt-in" approach and clear the introduction with both parties before you attempt to make the introduction at all.
With a double opt-in, you don't CC the person you'd like to introduce, and you provide context to both parties for why connecting would be beneficial to them in separate conversations. With this method, the pressure is off and anyone can back out without feeling bad or causing any issues. For good examples of single opt-in and double opt-in email introductions, check out the link below.