Pattern help

I am a new knitter, can someone please help me with a pattern question.
This is what the directions say:K21, SSK, (K2tog) twice, K to end.

Does this mean to slip 2 stitches knit those together and then knit two more together?

Also, the pattern says 2nd and all alt rows: purl

Does this mean just Purl all the stitches on the 2nd and all alternate rows or purl the stitches following the SSK instructions?

This is just a square that can be put together to make an Afgan.
Thanks for any help you experienced knitters might give.

Yes you are going to knit 21 sitches, slip slip knit together, knit two together TWICE, and then knit to the end. so you will have decreased three stitches in that row.

then you will purl every even row and do whatever the instructions say for the even rows.

Thank you! I still get a little confused trying to read patterns.