In sheet music, fortissimo is indicated by 'ff' showing a section should be played very loudly. Observe the 'ff' mark, that's your cue to play fortissimo.
Pro Tip 2/3
Emotional Intensity
Fortissimo often symbolizes a high point of emotional intensity within a piece of music. The soloist went fortissimo, expressing the song's passion.
Pro Tip 3/3
Orchestral Balance
Even in fortissimo, musicians must maintain balance; one instrument shouldn't overpower others. Even fortissimo, the flutes were harmonic with the violins.