Copyleft

"Copyleft is a play on the word copyright to describe the practice of using copyright law to offer the right to distribute copies and modified versions of a work and requiring that the same rights be preserved in modified versions of the work. In other words, copyleft is a general method for making a program (or other work) free (libre), and requiring all modified and extended versions of the program to be free as well." - Wikipedia

Copyleft is simply a contrast of copyright, which means it doesn’t restrict/limit the work to be reproduced, copied, and sold under another name. The work under copyleft license is under free to use, free to study and adapt, free to copy and share and also free to distribute modified version.