“(k1-yo-k1-yo-k1) in next stitch” is an increase technique where you increase multiple times into one stitch. This particular increase is an increase where you increase four stitches into one stitch for a total of five stitches. These types of increases are often used in lace patterns and to create bobbles or nups in your fabric.
(k1-yo-k1-yo-k1) in next stitch
[(k1-yo) 2 times, k1] in next stitch
(Knit 1 in stitch and without removing stitch from left needle, YO) 2 times, then k1 again and remove original stitch from left needle. 4 stitches increased in original stitch for 5 stitches