PAttern Question

What does this mean as it goes back down and then up in numbers what does it want me to do??

[B][U]136) Begin short rows. K 21.
135) Turn work (leaving last 21 stitches on the left
needle), k back over prior 20 stitches, sl1yf
136) K19[/U][/B]
137) Turn work, k 18, sl1yf
138) k17
139) Turn work, k16, sl1yf
140) k15
141) Turn work, k14, sl1yf
142) k13
143) Turn work, k12, sl1yf
144) k11
145) Turn work, k10, sl1yf
146) k9
147) turn work, k8, sl1yf
148) k7
149) turn work, k6, sl1yf
http://www.ravelry.com/patterns/library/jean-lafittes-mermaid-lap-blanket—sack

You’re working short rows to shape the tail here. Knit the given number of stitches and turn as you would at the end of a row. Don’t worry about the unworked sts on the left needle. You’ll get to them at the end when you work either the other side of the tail or bind off one of the fins.