The distance of an object is the total distance it travels, while the displacement of the object is how far away the object is from the starting point.
Because the body is moving in a STRAIGHT line, that means it does not change directions, therefore when the body gets to the destination, the total distance will be the same as the displacement. If the body were to change directions, then the magnitude of the vectors will need to be added up and calculated.
For example, let's say you are walking to your friends house directly across the street from your house. All you need to do is walk in a strsight line from the front of your house and you will get to your friends house. The distance and displacement will be the same.
Now if your friend lived right across the street, but 5 houses down, and you cross the street directly from your house then turn in another direction and walk straight, then the distance in this case will quite likely be greater than the displacement because the displacement is the distance from your house to your friends house when measured diagonally.
