sudo adduser jknoppf www-data and sudo chown -R www-data:www-data /var/www/html:
The first one will add the user jknoppf into the group www-data, and the second one will change the owner of all the directories and files, including /var/www/html, into the user www-data, and also change the group owner into the group www-data. Since the permission for /var/www/html is by default 775, and the user jknoppf is in the group www-data, this user can have full access to all the contents inside.
sudo chown -R jknoppf:www-data /var/www/html:
This command change the owner of all the directories and files, including /var/www/html, into the user jknoppf, and also change the group owner into the group www-data. Since the permission for /var/www/html is by default 775, and the user jknoppf is the owner of the directory, this user can have full access to all the contents inside.
Remarks: the permission 775 means:
First 7: the owner of the file have full access to files, i.e. read 4 + write 2 + execute 1.
Second 7: all users in the group, which the group is owner, have full access to files, i.e. read 4 + write 2 + execute 1.
Third 5: all other people that are neither the owner nor the members of the group only have access to read 4 + execute 1.