Meaning of Six degrees of freedom (6DoF) is how a body can moves in three dimensional free space. Any thing around us is free to move forward/backward, up/down,left/right. This means a body can translate in three perpendicular axes. In fig 1 it shows that forward/backward, up/down, left/right motion can be defined as +/- X, +/-Y, +/-Z
These motions are combined with rotation?of particular axes they name as pitch roll and yow Tilting forward and backward called pitch, turning left and right called yow, Tilting side to side called roll.
following are few example to understand better