Dear Student
A body moving at constant velocity can be described by a sum of velocities in two directions, typically x and y co-ordinates.
The path of a projectile may combine constant speed in the horizontal direction with acceleration due to gravity in the vertical direction.
Projectile motion ; motion of kite etc.
As in projectile motion body moves horizontally as well as vertically, it has velocity in both directions hence it is said to be in 2-D motion.
Regards