You can use a safety pin, paper clip, loop of different colored yarn, whatever will fit around the needle. Most markers are put on the needle to mark off a section of knitting where you might be doing increases or decreases. To `slip marker' is to move them from left to right needle when you come to them so they travel up your work as you knit. Sometimes you'll see a direction to put markers at each end of the row. These go in the end stitches and stay there, and are used so you can measure up from that point.