It's hard to understand complex licenses that is why all photos on Pexels are licensed under the Creative Commons Zero (CC0) license. This means the pictures are completely free to be used for any legal purpose.
The only restriction is that identifiable people may not appear in a bad light or in a way that they may find offensive, unless they give their consent. You should also make sure the depicted content (people, logos, private property, etc.) is suitable for your application and doesn't infringe any rights.
The CC0 license was released by the non-profit organization Creative Commons (CC). Get more information about Creative Commons images and the license on the official license page.
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