Gatsu the placeholder text inside of the token field tells you where you can find the token in your Matomo dashboard.
Personally I never used this feature, I prefer to access the Matomo dashboard directly. This is just a simple iframe to the same page you get in Matomo.
Filling in the token has no other purpose outside of enabling the iframe. It's not a requirement.
Matomo is the new name of Piwik. The extension might still say Piwik, I haven't checked.