Hi

Kindly find my response for your inquiries below:

I have set up a cron job but i do not understand why I have to do this.

-> Cronjob is a schedule command/task that is executed on your server to run automatically at a specified time and date. A cron job is the scheduled task itself. Cron jobs can be very useful to automate repetitive tasks. So you no need to run the command by yourself and it make your site more optimize and less heavy to access

How can I send these emails, and what needs to be done to make sure they go out as intended?

-> To make sure the email send out to subscribers, you have must make sure the cron job has setup above was working/run correctly as well.

And Can i resend these?

-> Yes, you can resend it by(backend):
1. Posts>posts>click on the mail icon : https://www.screencast.com/t/Ncduia6LH