The two basic dynamic indications in music are piano, meaning "softly" or "quietly", usually abbreviated as p; and forte, meaning "loudly" or "strong", usually abbreviated as f. More subtle degrees of loudness or softness are indicated by mp, standing for mezzo-piano, and meaning "half-quiet"; and mf, mezzo-forte, "half loud".
Beyond f and p, there is ff, standing for "fortissimo", and meaning "very loudly"; and pp, standing for "pianissimo", and meaning "very quietly". To indicate even more extreme degrees of intensity, more ps or fs are added as required. fff (fortississimo) and ppp (pianississimo) are found in sheet music quite frequently, but more than three fs or ps is quite rare. It is sometimes said that pppp stands for pianissississimo, but such words are very rarely used either in speech or writing, even when present in a score.
There is some evidence that this use of an increasing number of letters to indicate greater extremes of volume stems from a convention dating from the 17th century where p stood for piano, pp stood for più piano (literally "more quietly") and, by extension, ppp indicated pianissimo. Antonio Vivaldi seems to have written using this convention, but it was largely replaced by the above, more familiar, system by the middle of the 18th century.
In addition, there are words used to indicate gradual changes in volume. The two most common are crescendo, sometimes abbreviated to cresc, meaning "get gradually louder"; and diminuendo , sometimes abbreviated to dim, meaning "get gradually softer". Signs called "hairpins" are also used to stand for these words. These are made up of two lines which connect at one end and get gradually further apart. If the lines are joined at the left, then the indication is to get louder; if they join at the right, the indication is to get softer. In other words, the further apart the lines, the louder the music. The following notation indicates music starting moderately loud, then becoming gradually louder and then gradually quieter:
It should be noted that dynamic indications are relative, not absolute. mp does not indicate an exact level of volume, it merely indicates that music in a passage so marked should be a little louder than p and a little quieter than mf. Tchaikovsky indicated pppppp in a passage of his Symphony No. 6, but the music would probably not be played any quieter than p would in other pieces, or even in different parts of the same piece.