Dynamic marks indicate how loud or soft the music should be played. For instrumental parts, dynamic marks are placed under (or near-by) a note beneath the musical staff. For a vocal part, the dynamic mark is placed above (or near-by) a note on top of the musical staff.

forte possible: As loud as possible
fortissimo: Very loud.
forte: Loud.
mezzo forte: Moderately loud.
mezzo piano: Moderately soft.
piano: Soft.
pianissimo: Very soft.
piano possible: As soft as possible.
forte-piano: Loud, then immediately soft.
sforzando: Strongly accented.
rinforzando: Reinforced.