A couple of people have asked about parking ... this is just a way of having 2 or more colors going at the same time. Let's say you have some red stitches to do in rows 1 and 3 with some other color(s) between them in row 2. So you do the stitches in row 1, then come up at the beginning of the first stitch in row 3 and leave the needle and thread hanging there on the front of the fabric while you do the intervening stitches in row 2.
It really only applies IF you have an area where a color has several stitches in close proximity but not continuously, AND if you would rather stitch without leaving holes and filling them in afterwards. I've tried it both ways and it seems like when I go back to fill in a hole it is hard to get the needle through all the weaving on the back and as a result the stitch doesn't look quite as neat. But I think my weaving leaves a lot to be desired.
If you forget what color you have on the "parked" needle you just look up that stitch on your chart.
I have sometimes had 4-5 needles parked at a time. That seems to be the limit of what I can keep out of the way of what I'm currently stitching.