A CAD program is a software used to aid in the creation, modification, or optimization of a design.
CAD stands for Computer Aided Design. It is the use of computers to aid in the creation of a design. CAD programs/software enable designers, engineers (basically anyone who makes), to design, inspect, analyze in an integrated user interface on a PC or Mac.
There are a bunch of free software available for download or used in a browser that can get you started.