What is an Opt-In?

When someone would like to sign up for your office newsletter, they have to "opt in," which essentially gives the office permission to proactively contact them. This can be found in a couple places on your profile. You can find this in the righthand sidebar of a constituent's people record: 

Or you can find it on the profile tab of the people record under Yes/No Fields:

Opting out means they do not want to receive the newsletter at all. 

No preference means they have not specified whether or not they would like the newsletter.