Very slow, easy pace.
The pace you could keep for an hour or even more, but it’s not exactly easy.
A hard but controlled effort you are able to continue for for 30-45 minutes.
The pace you'd keep if you were racing or testing yourself over a distance of 5km.
The fastest you could run for between 1 and 5 minutes.
You adjust this depending on the length of interval.
Basically, the hardest you can run for the time, distance and repeats specified.
The fastest you could run for 1 minute, without injuring yourself.