Difference between a make 1 and cast on in the middle of the row

I have done M1 through out this pattern. Now it calls for knit 2 then cast on 2 stiches. what is the difference. and how do I cast on after knitting 2 stitches. thanks

Casting on 2 sts will make a little gap, like for a buttonhole. K2 then move the right needle to the left hand, use a knit or cable CO to add the 2 sts, put the needle back to the right hand and finish knitting the row. If you’re not familiar with thse COs, there’s videos on the Cast on page here.