Disclaimer: I am not a lawyer, and the best person to decide whether you can use software in a production environment is you or a lawyer on your behalf.
The following licenses apply
A license file will be part of any downloaded package or distribution. Apache License Version 2.0 is a permissive free software license commonly used which, according to Wikipedia:
allows the user of the software the freedom to use the software for any purpose, to distribute it, to modify it, and to distribute modified versions of the software, under the terms of the license, without concern for royalties.
NOTE: X-Pack is proprietary software developed by Elastic that falls under a commercial license agreement and not Apache License Version 2.0; please get in touch if you'd like to know more.
Hope that helps!