Help with new cast on

I’m knitting a shawl- can someone explain the cast on to me in knitting terms! I’ve never done this before:

Cast On: With length of waste yarn and crochet hook, chain
4 stitches. Using knitting needle and your shawl yarn, pick up
and knit 2 stitches in the bumps at the back of the 2 middle

Garter tab: Knit 7 rows in garter stitch (knitting every row).
At the end of the 7th row, do not turn. Instead, rotate work 90
degrees, pick up and knit 3 stitches in each of the garter
bumps, then release the 2 stitches held by the waste yarn and
knit both stitches. [7 stitches now on the needles]

Why am I using waste yarn? I’m confused!

This is one way to do a provisional cast on. You can use waste yarn because you’re going to pull it out, leaving live sts of the working yarn. The waste yarn can then be thrown away.

You’re knitting a tab for the beginning of a shawl, similar but not exactly like this one.

Thank you!