Cables are twists in the knit fabric formed by working stitches in a different order from the way they usually present themselves on the needles. They're typically made by slipping some number of stitches off the working needle, holding it to the front or back while working other stitches, then working the held stitches.
When the stitches are held to the front, in what's known as a cable front, the resulting cable with slant to the left.
So when you work a cable back, the stitches are held to the back of the work, which makes a right-slanting cable.
The instruction is usually seen as an abbreviation, such as C6B, which means cable 6 back.
To form a cable 6 back, you would slip 3 stitches purlwise onto your cable needle, then hold the cable needle to the back of your work while you knit the next 3 stitches off the left-hand needle.
To finish the cable, knit the 3 stitches from the cable needle onto the right-hand needle. This will produce a right-slanting cable.
Of course that example describes the most basic way to form a cable, and there are many others, but just as knowing how to knit and purl will get you far in basic knitting, knowing the moves of cable front and cable back will go a long way toward getting you comfortable with cable knitting.
And all you need to know about the cables you are working for a particular pattern should be explained in the pattern itself, either on the first reference to a particular type of cable or in a key that explains how all the different cables in the project should be worked.
It may happen that you decide you prefer the look of one type of cable over another or that you find either the cable front or cable back easier to do.
I happen to prefer working cable fronts -- I just find it a little easier to control the needles that way -- and if I wanted to knit a pattern that was written to have right-twisting cables, I might just work left-slanting cables anyway because that's what I prefer.
You're free to do that as well, of course, so long as changing a cable's twist doesn't significantly change the look of the finished piece (such as if there were cables with mirrored twists and you decide to work them all the same way, which is also fine, just think about it before you decide to do it).
Cabling without a Cable Needle
One of the problems some people have with knitting cables is that they never seem to have a cable needle handy when they need one. You can also use a double-pointed needle that's around the same size as the needle you're knitting with, but another option is to forego using a cable needle altogether.
The full instructions are in this tutorial on doing a cable back without a needle. It's not that difficult to do and once you've got it down you will love the ease of being able to work cables without any extra equipment.
Cable Knitting Patterns
Ready to try out a cable back on your own? Try my cabled napkin rings, which you can make all as right-twisting, left-twisting or a mix of cables to learn both techniques quickly on a fast little project you can make a bunch of in no time.
Or work the Cabled Headband holding the stitches at the back instead of the front for another quick, easy and really useful project.