That is the command to enable the apache2handler (libapache2-mod-php)
The php-fpm is another SAPI, which is mutually exclusive with apache2handler.
You should understand how php functions and completely remove (purge) the libapache2-mod-php* packages if you want to use the php-fpm SAPI
Run the php-updater script and read its help for more details.
Much and good luck,
ernolf