Thanks for all of you. I have understood it.

Just making a summary.

In September 2007 Morton decided to stop maintaining the -mm tree. In
February 2008, Stephen Rothwell created the linux-next tree.[1]

The linux-next tree is a staging trees, where subsystem trees are
collected for testing and review.[2]

Linux-next tree holds code that will go into the mainline during the
next merge window.[3]

Stephen said, about 90% of the code that shows up in the mainline
during the merge window was in linux-next prior to the opening of the
window.[3]

[1]:https://en.wikipedia.org/wiki/Linux_kernel
[2]:http://www.linuxfoundation.org/content/how-participate-linux-community-0
[3]:https://lwn.net/Articles/608933/

Other materials:
https://lwn.net/Articles/608917/
https://lwn.net/Articles/571980/
https://lwn.net/Articles/287155/

Regards,
Hao Lee