sadly there aren't magic numbers. some people need to really restrict carbs to shed fat at a good pace....others don't.
some tolerate a lot of protein, others don't.
we can speak in general terms, but at the end of the day, you'll just have to track what you eat, and track your progress and then tweak as necessary, track again, note progress, and tweak if necessary.
Interval cardio means you don't just keep the same pace the entire cardio session. Using jogging as an example...you wouldn't jog a steady 5mph for 20 mins...instead you'd start at a brisk 3.5mph walk. then you'd run at 6-7mph for a few mins, then slow back down for a few mins, then run again.
HIIT is just a more intense interval cardio method, where you basically run 'balls to the wall', then cool down, repeat. its more formulaic than that.
but essentially, instead of plodding along at a pace you can maintain for the cardio session, kick it up several notches, until you can't maintain, then take an appropriate breather...and repeat.
most all gym grade cardio machiens have interval workouts build in, that increase speed, resistance and/or incline.
riding a bicycle up and down hills would be 'interval style' ...vs. flat road biking, which'd be considered 'steady state' cardio.