These messages appear because Google Chrome requires permission for extensions to do these actions. On other browsers, like Firefox, the extensions do this by default, thus you don’t see these messages.

In this case, the data that is being changed is the advertisements you see, or to be more precise, do not see.

It can:
Read and change all your data on the websites you visit
Display notifications

Before I click on “Add extension” I want to know who is reading and possibly changing my data on the websites that I visit?

Any advice would be greatly appreciated as this piece was omitted from the instructions above.

Thank you