I'm new and I need Help!

Hi - my name is Cathy and I’m an old new knitter. My mom taught me to knit many many years ago when i was a young girl. Now that my own kids are grown, I decided to take up knitting as a new hobby to keep me busy. I’m making a baby hat, very simple pattern, but the end is what I don’t understand. I’m used to casting off at the end of a pattern, this pattern says:

Break yarn, thread through stitches and seam side. What exactly does this mean? Thanks in Advance for any help

Cut the yarn but leave a long enough tail to pull trhough the sts like a drawstring, then sew the edges together.