When you bind off stitches like usual over consecutive rows, you will get steps between the bound-off stitches. Those steps are not practical when you need to sew pieces, or pick up new stitches along the edge…
The sloped bind-off will give you a steady bind-off edge over multiple rows, without any steps.
In this tutorial, I bind off 5 stitches every RS row.
Knit 2 sts. Pass the 1st stitch on right needle over the 2nd stitch. You bound off 1 stitch.
Continue until you have bound off the right number of stitches for this row (here, 5 stitches), then knit to end of row.
Next row (WS), purl to 1 stitch to end of row.
Turn. You have 1 one stitch on right needle (not worked from the previous row).
Slip the 1st stitch from left needle onto right needle, knitwise. You have 2 stitches on right needle.
Pass the 1st stitch from right needle over the 2nd stitch. You bound off 1 stitch.
Continue to bind off stitches (as usual) until you have bound off the right number of stitches (here, 5 stitches). Knit to end of row.
Repeat steps 2 to 4 as many times as necessary. You’re getting a slopped steady edge without steps.