Sock Question! Help!

I am struggling along on the toe-up sock pattern, and have more or less finished the toe, unzipped the provisional cast on, and have close to the right number of stitches divided among 4 needles. Whew! Now the pattern says to “work straight” until the foot is about 2 inches shorter than the desired finished length. Place 40 instep stitches on one needle and 40 heel stitches on another needle. So my first question is : what does work straight mean???
And am I supposed to know which among the stitches is the instep and which is the heel??? (Obviously I have never knitted socks before!!!)

Work straight just means that you are going to work in whatever stitch pattern you have been, but with no more increases or decreases.

The instep stitches will be the 40 on top of the foot and the heel stitches will be the 40 on the bottom of the foot.

work straight means that you knit with the same number of stitches. If you’re knitting with a pattern (lace or rib or whatever) then you just work the pattern, but keep the same number of stitches. If it doesn’t have a pattern, then you just work stockinette with the sam number of stitches.

the instep stitches vs. the heel stitches.
the heel stitches are the ones that make up the heel and the instep ones are the ones that go across the front of your ankle.

you can choose which is your heel and which is your instep, but the top half of the stitches from the toe makes up one (instep or heel) and the bottom half of the to makes up the other (heel or instep).

Do I mark the groupings of stitches in some way at this point to differentiate between heel and instep or doesn’t it matter? ( I just finished the toe part) And if I just keep knitting, its like using circular needles and it comes out as stockinette, right?

When you get to that spot, you can put a marker at each side between those sets of stitches if you like.

And yes, all knit stitches in the round whether using dpns or circulars, makes stockinette stitch.

Thanks so much for the help. I’ll post a pic when I finish!