I will try to explain in very simple words so that you can undertsand better,

3D printing is bascially a way of creating a 3 dimensional  solid objects ,where

3D  printing is done by building up the object layer by layer. Usually, 3D printers use plastic, because it is easier to use and cheaper.

coming to nxt topic

WHERE DOES ITIS USED IN AEROSPACE ENGG

the aerospace industries uses 3d printing in end to use parts ,prototype

alleviate supply chain constraints,limit warehouse space and etc