What is Java Communications API? Why do we need it?
I think best answer for these questions is the Oracle’s answer. So, here it is: “The Java Communications 3.0 API is a Java extension that facilitates developing platform-independent communications applications for technologies such as Smart Cards, embedded systems, and point-of-sale devices, financial services devices, fax, modems, display terminals, and robotic equipment.” 
Eventhough they have the best answer, Oracle seems like lost it’s own product’s link to provide developers. I had to download an old version from MIT’s server. You also can download it from here: http://llk.media.mit.edu/projects/picdev/software/javaxcomm.zip
Anyways, to install it Netbeans, we should do followings step-by-step:
1. Right click on your project and select “Properties”
2. Click on “Libraries” at left side of new window then click on “Add JAR/Folder” button
3. Go to the directory which contains comm.jar file
4. Click “Yes” on the pop-up window. There will be new folder generated automatically on your system. DON’T DELETE THIS!
5. Now you must see comm.jar under your project