When you run IP warming plan, generally its required to select few email templates that can be sent to IP Warming audiences.
As you need to achieve the peak volume in your IP Warming plan and the volume will grow gradually, there will be scenario, where we have to use same audience to send the email again, to reach the peak volume. For ex last few days when last day you send it to peak volume and days before will also need higher audience size.
Hence, IP warming plan is required to be separated in to phases which would essentially need same email to be sent and Runs would mean the audience that it should be sent to without repeating it to same person.
So in a summary
All runs in a phase can have same email creative and one person will only be a part of one run in that phase. i.e he receives only one email in a phase
Different Phases would usually have a different email creative and same person can be part of multiple phases and can receive email in all phases.
Using phases, system will automatically make sure, same person is not sent email more than once.
And it becomes easy during execution as well, in case you go into issues with ISPs, to break the phases.