Until now, we have not bothered with which direction any motion was in.  Very soon, however, it will matter very much.

Let's say you walk 10m North.  Your displacement from the start point is 10m.

Simple enough.

Next, you walk 15m South.  Your displacement is 5m.

Huh?

While wlaking the 15m South, you crossed over your start point and went another 5m.

You will often see one direction chosen as "+" and the other as "-".  In the case above, if we chose North as positive, our answer for overall displacement is -5m.
Inversely, if we had chosen South as positive, our answer would be +5m.

Displacement=DX= Xfinal-Xoriginal .  The triangle is the Greek letter delta and stands for "change in".
Distance and displacement are different.  Distance is just how much ground has been covered.  Displacement is how far something is from some starting point.
So, if you run one lap, your distance is one lap and your displacement is zero (because you wind up back at start point)