SS is indeed steady state but rather than constant pace is the training we do in the aerobic zone- which tends to be the highest volume and is in contrast to anaerobic which is most often intense intervals or a TT ( time trial - or all out effort).
Pace as you know is time/500m. People often talk about a pace vs some known reference. In Iain’s example from the PetePlan he was saying that a (hard) 4x2k interval session might be attempted at your Steady State (aerobic) pace - (minus) 10 seconds. So if aerobic SS is @ 2:25pace then try the 4x2k at 2:15.
Intervals are written as 4x ( number of reps)
2k ( distance of each rep)
5r ( rest length in minutes or eg 90” would be 90 seconds)
If the r is written before the number then it’s the stroke rate - eg r20 = 20 strokes per minute.
HTH