All free HDRIs on this site are licensed under the Creative Commons CC-BY license.
This is a sort of open-source/copyleft license, meaning that you're welcome to:
- Use them for any purpose,
- Modify them in any way,
- Redistribute them anywhere...
... as long as you give credit to the original author (i.e. mention something like "The HDRI used here is from hdrihaven.com").
If there is no suitable place for you to mention the credit (e.g. if you use it in an advertisement), please rather buy the full-resolution version of that HDRI which is under a separate Royalty Free license and does not require attribution. It's only $5.95 after all ;)