Markers are a way to mark where to do certain stitches or techniques, or to help you count!
There are two different types of markers: ring markers (little rings) and locking markers (little safety pins). There are many different styles of each type.
Ring markers are put directly onto your needles, in between stitches, while you work. You put them onto your right needle then continue working, then when you get back to them, you slip them off the left needle and onto the right needle and continue working again. They cannot be taken off the needle unless you knit to where they are on the needle.
Locking markers can be used for the same purposes as ring markers, but they can also clip onto any part of your work, or be taken off without you having to knit to them. They can be used to mark the right or wrong side of your work, or to hold dropped stitches before you can fix them.
In the videos below we are using a ring marker.
m - marker
pm - place marker
sm - slip marker
rm - remove marker
You don't really need a video for this technique; simply take the marker off your needles and set it aside!