All Photos on Pexels can be used for free. While the most photos are released under the Pexels license, some photos are covered by the Creative Commons Zero (CC0) license. Both licenses are similar and allow you to do the following:
All photos that are covered by the Creative Commons Zero license are marked with "CC0" on the photo page. The CC0 license was released by the non-profit organization Creative Commons. You can get more information about Creative Commons and their license on the official license page.