If I’m not mistaken, it means 6 stitch right cross, and it is just describing your cable (ie sl next 3 sts to cn, k3, k3 from cn) so they don’t have to type it all up again every time. It is very confusing to do it this way, rather that write it up at the top of the pattern.
I could be wrong, but with the little bit of instruction you wrote, that is what makes the most sense to me. If you do think I’m right, then don’t do it twice, just ignore it in bracket, but remember that’s what it’s called when it comes up again. Hope that makes sense.
The part I put in bold italics is what constitutes the “6-st rc”. It stands for 6 stitch right cross. So everytime you see “6-st rc” in this pattern, do this:
Slip the next 3 sts to a cable needle, and hold them at the back of your work.
Now, pulling to avoid a gap, knit the next 3 stitches as normal.
Now, knit the 3 stitches from the cable needle.
So what you’ve done is take 3 stitches and crossed them to the RIGHT over 3 other sts. Some patterns will call this a Right Cable, and some will call it a Back Cable, so you might see 6RC in some patterns, or 6CB in others, or variations-- like this one calls it 6-ST RC.
Have you have done a cable before? A cable needle is a very short needle that either has a bend in it or a textured part in the middle of it to hold the stitches. It’s a little awkward until you get used to it, but cabling produces beautiful results.