That’s great news you finished the other leg.
I think before you were making the backs and now you have moved on to the front.
If you look at the picture the front panel does not decrease until it measures 26-27cm so for some time you are working a straight rectangle.
The instructions are where the centre of this piece is patterned with rib stitches and the outer pattern is stocking stitch. The rib is gradually changed to st st to form a triangle shape - but with same number of stitches throughout.
You have the first few rows all in rib. Then you worked 3 rows where the first 3 stitches are stocking stitch and the centre long stretch is rib.
Row 4 has 5 stitches each end in st st
This means the rib has reduced by 4 because the st st increased by 2 at each side (changing from 3 st st in rows 1,2,3 to 5 st st in row 4)
No increases or decreases, just stitch changes.
So, see how you had 3 rows with 3 st st at each end? You will do 3 rows with 5 st st at each end (row 4 is the start of this group of 3 rows,so rows 4, 5, 6).
It says do this change of stitches 4 times more, on following 3rd row:
3 rows with 7 st st each end (rows 7,8,9)
3 rows with 9 st st each end (rows 10,11,12)
3 rows with 11 st st each end (13,14,15)
Row 16, 13 st st each end
Then it says to do the stitch change on every alternate row, 18, 20, 22, 24…
Until you have mostly st st and have just 3 rib stitches left, work 2 rows then change to all st st.
Does it make sense?
Remember to look at your fabric to see where your ribs are to keep your knits and purls in line and keep the correct rib pattern.