Public Tweets (the default setting):

These are visible to anyone, whether or not they have a Twitter account.

Protected Tweets:

Only visible to your Twitter followers. Please keep in mind, your followers may still capture images of your Tweets and share them.

When you protect your tweets, you’ll receive a request, when new people want to follow you, which you can approve or deny. Your Tweets, including permanent links, to your Tweets, will only be visible to your followers. Your followers will not be able to use the Retweet icon to Retweet or Retweet with comment.

Protected tweets will not appear in third-party search engines (like Google search). Your protected Tweets will only be searchable, on Twitter ,by you and your followers. Replies you send to an account that isn’t following you will not be seen by that account (because only your followers will see your Tweets).

More information can be found here: https://help.twitter.com/en/safety-and-security/how-to-make-twitter-private-and-public