swimming set question

20 repetitions of 100 yards/meters with 15 seconds rest is probably what it means.

Where did you get the workout from?
Is there any recommendation on how fast/hard to swim the actual 100s?
What kind of a swimmer are you?

If you are a true newbie, that's a stiff swim workout. What plan are you using?

Dropping 2k in one set for a newb is quite a bit. If you are truely new, I wouldnt push past 1500 yds for an entire workout for the first couple times.

As far as I know, and I'm no fish, the number after the @ symbol is usually the interval that you leave at, but 15 for a 100 makes no sense so AB is probably right and it's supposed to be the rest interval between swims. That gives you no prescription for speed though, other than to realize that with 15 seconds rest you might push near or just over your threshold pace but you wouldn't have time to recover from really hard sprinting efforts. Also, as the others have mentioned, 2k is likely too big a set for a beginner especially if done pushing over threshold pace.