There are two types of displacement you could be referring to in science. One is a measure of how far away an object ends up from where it started out. You will use this in physics problems when you are calculating velocity. Velocity is the displacement from its origin of an object, divided by the time; it is therefore a vector quantity, because the direction matters. This is different from speed, which is distance traveled divided by time. You can see the difference by picturing yourself running around a 400 meter track and back to the start. You will have a speed of 400m divided by however many seconds you ran; if 400 seconds, then 400m/400s for a speed of 1m/s. Your velocity, however, is zero because your displacement is zero--you ended up at your starting point.
The other use of "displacement" in science refers to measuring the volume of an object by seeing how much liquid it displaces when it is submerged. For example, if you fill a large graduated cylinder to a volume of 600 ml, and found that after you submerged your object the total volume went up to 750 ml, you would know your object's volume was 150 ml. This is a useful way to find the volume of an oddly shaped object.