Swim session question

It's the total time between 100's.  If you do your first 100 in 1:45 you'll get a 30sec rest before your next 100.  After your for 10 you'll start doing them on 2:20's etc...

 Hope this helps.

"10 on 2:15 "

This means that for each 100, for these 10 you have 2:15 to complete the 100, if you get done before 2:15, the remaining is your rest time.  It's called a sendoff.

More on reading swim workouts for the silver/gold plans at:

http://www.beginnertriathlete.com/cms/article-detail.asp?articleid=630