is a stitching technique which makes your stitching neater by not leaving "holes" between rows as you stitch, and faster because you don't anchor floss and thread a new needle as you change colors. A "hole" is a spot not stitched (yet), wholly or partially surrounded by completed stitches. Going back and inserting the stitch into a hole is more difficult than if the adjacent stitches were not already in place. (You can avoid leaving holes without parking, but then you have to anchor colors each time you finish a contiguous section, which can make the back messier in areas with a lot of color changes.) Here's how to do it.
1. Let's work through this very simple chart. It only uses 3 colors, and has 15 stitches.
2. Most people stitch left to right and top to bottom, so start stitching the A's, which are red. After the
first row of A's, the next A is in row 3. If you make that stitch now, it will be harder to insert the first stitch in the second row neatly, so park the red floss. Come up at the beginning of the A stitch (as if you were going to make the stitch now) but then just leave the floss and needle hanging on the front of the fabric. (We are assuming that you cross by coming up at the lower left, down at the upper right, then up at the lower right and down at the upper left.) Don't worry, the needle won't fall off.
3. The next symbol in the chart is B, which is blue. Start stitching the B's (it's OK to leave gaps between stitches in the same row -- see * at the end). Do all the B's in row 2, but then notice that while stitching the first B in row 3 will not leave a hole, stitching the second one will. You could make that first stitch and then park the blue floss, but for this example we'll park it now and wait to do both the remaining blue stitches together. So come up at the beginning of the next blue stitch and leave the floss and needle hanging.
4. The next symbol in the chart (you're working on finishing row 2) is C, which is yellow. You can complete all the yellow stitches without leaving holes, so do that. This is the end of C for this little chart, so anchor the floss in back and snip off the remnant.
5. Now we are ready to continue with the first parked color. In a more complicated chart, you might not know what the symbol for this color is, so refer back to the chart. You've been highlighting completed stitches (haven't you?), so it's easy to compare the chart to the stitching. The parked thread is for the first stitch in row 3 and that's A.
6. This is the last A in the chart, so after you make the stitch you'll anchor the floss and cut off the remainder, and your work looks like this. Refer back to the chart to determine the symbol for that other parked color (stitch 3 in row 3) and it's a B, and so is the stitch next to it. Complete those stitches, and that's the end of this chart.
In a more involved chart, when you came to a parked color, you would probably make one to several stitches with that color, then park it again farther along, leapfrogging down the chart. The floss you carry along the back should be at least partially covered by the intervening stitches, so you may be willing to skip farther than you might usually do.
You'll obviously need a needle for each color you are going to park. Some people limit parking to background areas, where there are only 3-5 colors mixed together. Other people have no trouble managing 100 parked colors.
* The reason that horizontal gaps are OK while vertical ones are not is this. When you fill in a gap between 2 stitches in the same row, you are bringing your thread up through holes that only have one stitch in them so far. If you leave a hole surrounded by other stitches, when you fill it in, you have to get your needle past up to 3 other stitches without piercing the existing floss, which is much harder to do.