Technically, what it does is make a call to the Discourse sync_sso route and passed their data (WordPress user_id, username, name, email…) to Discourse immediately after they login to WordPress. Details about the sync_sso route are here: Sync DiscourseConnect user data with the sync_sso route.
The only side effect of this that I’m aware of for WordPress users who have never visited the Discourse site is that they will start receiving digest emails from Discourse.
This is why you want to encourage your Discourse users to register on WordPress with the same email address as they are using on Discourse. As long as the email addresses match, they will be logged into the correct Discourse account. This is assuming you take care of the issue with email verification that we discussed in previous posts.
It’s likely that you’re going to end up with some users signing up on WordPress with different email addresses than they’ve used on Discourse. For that case, a new Discourse account will get created for them, using the WordPress email address. You’ll need to manually merge the old Discourse account into the new Discourse account: Merge User Accounts.