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 pictures are free for personal and even for commercial use.
You can modify, copy and distribute the photos.
All without asking for permission or setting a link to the source. So that attribution is not required.
The CC0 license was released by the non-profit organization Creative Commons (CC). Get more information about the license on the official license page.

Restriction
Identifiable people may not appear in a bad light or in a way that they may find offensive, unless they give their consent.