What is the meaning of Ff?

fortissimo

"and following" (pages, etc)

ISO 639-1 language code for Fula.

Often written in old manuscripts or transcriptions of them, denoting a capital F.

Initialism of forfeit.

ff.

abbreviation of effen; briefly, just

The ninth letter of the Welsh alphabet, called èff and written in the Latin script. It is preceded by f and followed by g.

Source: wiktionary.org