Now more than ever, you can easily resend a broadcast to those that didn't open the first time, or to those that have been added to your list (or product or whatever), since the last broadcast.
Go to a Broadcast that you've sent:
Both will create a duplicate of the broadcast, with the same recipients, but with one additional twist:
"Resend to missing" will exclude anyone who received the first broadcast.
"Resend to unopens" will exclude anyone who did not report as having opened the email, or who didn't click a link in the email.
Unopens are tricky because the way we know if they opened the email is if they requested a hidden 1-pixel image inside the email. Depending on the email client, though, that method can be very unreliable. That's why we also exclude anyone who clicked a link, since, obviously, they, too, opened the email, even though they may not have requested the image. There will be people who did open the email and didn't click anything, who will get the email again, but that's the best we can do.
When resending to unopens, one common tactic is to change the subject line and leave the body the same. The subject line didn't make them open the last time, so maybe a new subject line will? If you're super-savvy you might even know what percentage of your audience responds to different styles of subject lines, ie. curiosity-based, vs. FOMO-based, vs. "towards" motivation, vs. "away from" motivation, and so on.
On a final note, using the "resend to missing" will not re-drip content that you released in a broadcast.