Cardio - sustained, steady-state, low-to-moderate intensity exercise commonly call "cardio" or more scientific... aerobic exercise, cardiovascular exercise, etc.
High-intensity - two options
1) Super High Intensity with lots of rest in between - like sprints (usually 10 sec or less) and rest.
2) High-intensity cardio or intervals where you go hard for a short period of time (30 seconds to 2 minutes) followed by a period of active recovery at an easy pace. Running fast, light jogging, running fast, light jogging, etc.
Cardio is a good thing to do early in the off-season to build a base of aerobic fitness. Outside of that, for softball performance, doing more cardio or too much of it is useless and might even be a waste of time because we don't rely on that energy system.
Super high-intensity or speed work (sprints) is what you should do. Every action in softball is under 10 seconds. This is what every player should be doing - speed work at a high intensity with rest in between to recover.
As for intervals, it is a very effective way to do "cardio" or to go beyond plateau but it is not for beginners, start with regular steady-state cardio then progress to intervals.
For building endurance - both protocols are good for endurance needed for softball.
However, for out-of-shape people, you always want to start with steady-state aerobic work. In plain english, good old fashion cardio where you run, ride a bike or do work on a cardio machine for 20-30 minutes at pretty much the same pace throughout. The pace should make you work without being killing you.
The first thing is to build endurance (foundation), so you do steady-state for 4-6 weeks, all the way to the way where you can sustain it for 30-45 minutes at a moderate pace without feeling totally drained.
Then, you need to move to intervals. Doing intervals for beginners or out-of-shape is not really recommended unless you low-intensity intervals.
Build endurance first with steady-state, then do intervals.