Dynamics are indicators of the relative intensity or volume of a musical line.
Other commonly used dynamics build upon these values. For example “pianississimo” represented as ‘ppp‘ meaning so softly as to be almost inaudible, and fortississimo, (‘fff‘) meaning extremely loud. In some countries, use of this dynamic has been virtually outlawed as endangering the hearing of the performers.
A section of music in which the music should initially be played loudly (forte), then immediately softly (piano). Another value that rarely appears is niente, which means ‘nothing‘. This may be used at the end of a diminuendo to indicate ‘fade out to nothing‘.
A small “s” in front of the dynamic notations means “subito“, and means that the dynamic is to be changed to the new notation rapidly. Subito is commonly used with sforzandos, but all other notations, most commonly as “sff” (subito fortissimo) or “spp” (subito pianissimo).
Dynamics with 3 letters (i.e. ppp & fff) are often referred to by adding an extra ‘iss‘. This is wrong in the same way as ‘loudestest‘ is in English (There is no word like Loudestest in English; this means fff is impossible unless the players wouldn’t mind to get their ears damaged. But ppp could be possible because it will be close to silence but not completely silent, but here, the players would find it a bit difficult to sound a clear/correct note).