Original: Code licensed under the GNU GPL can be used in other projects, so long as they also are released under the same or a compatible license. In order to make the Linux available for commercial use, Torvalds initiated a switch from his original license (which prohibited commercial redistribution) to the GNU GPL.
Simplified: To make the Linux kernel suitable for use with the code from the GNU Project, Linus Torvalds started a switch from his original license (which did not allow people to sell it) to the GNU GPL.