Sunnah means the way of the Prophet salla Allahu alaihi wa sallam. This is the linguistic meaning which means that the 5 daily prayers are Sunnah and Hajj is also Sunnah. But when we want to talk about the ruling of a particular thing, we have to classify it under the 5 categories which are: mandatory, voluntary, permissible, makrooh (not preferred) and forbidden. Growing the beard is mandatory because the Prophet salla Allahu alaihi wa sallam ordered it and the 4 schools of thought say that it is forbidden to shave it.
And Allah knows best

A close friend who will be leaving for her home country soon, wanted to know about the ruling of growing the beard for men. Is it haraam for them to shave? If yes then how can it be haraam if it is sunnah & not mentioned by the Prophet as fardh?