Help with a pattern

Hey there,
I’m a newbie knitter and I’m having trouble figuring out what this all means:

Next row (RS): Knit 21(21, 19) sts, yo, k1 (twice), yo2, k1 (three times), yo3, k1 (4 times), yo4, k1 (5 times).

I know that the brackets mean you do what it says in front of the brackets as many times as it says inside the brackets, but why are there 2 numbers?

i know this is probably pretty easy for anyone who know’s what they’re doing but I’m stuck!
Thanks a lot!

The first set of number - k 21 (21, 19) are what you knit at the beginning according to which size you make. Then yo, k1 2 times, 2 YOs and k1 3 times, yo 3 and k1 4 times and yo 4 and k1 5 times.