If on a round trip you travel $6\ km$ and then arrive back home :
(a) What distance have you travelled ?
(b) What is your final displacement ?

(a) Distance traveled $=$ total length of the path$=6\ km+6\ km=12\ km$

(b) As going on a round trip of $6\ km$ and then come back, there is no change in position. So the displacement is zero.