./certbot means running it from the current directory. There are differences in all of the documentation and tutorials in terms of sample commands depending on how, and in some cases where, Certbot was installed on the system.
If you installed via an operating system package, using a package manager, you can probably use the
sudo certbot forms. If you installed using
certbot-auto, you have to
cd to the location whether you originally downloaded
certbot-auto before running
./certbot-auto (in that particular directory).
Anyway, it seems you said that the
sudo certbot form works for you, which suggests that you got it via an official operating system package, and you're wondering about the webroot directory?
It should be the same directory where you would create or edit website content for the top level of your web site. Do you know where that would be?