PHP is the programming language WordPress is built with.
It’s constantly being updated and improved, and these updates come out in the form of versions.
The higher the version number, the more recently it was released.
With each release, the performance gets a little bit better, so you always want to be on a recent version.
But be careful, as the latest version can sometimes cause issues, so make sure to test this out on a development site before you update your live website.
To change your PHP version, just google “How to change PHP version [name of your host]”, and you’ll find step-by-step instructions for your host.