Each button adds the indicated mark to a selected note or group of notes; the center button changes the note back to its "plain" form. These can be combined where reasonable; for example it's possible to combine staccato and tenuto marks to indicate "stressed and moderately short." These expressive marks sometimes have an imprecise meaning; following is a list of typical interpretations:
Played somewhat shorter than the notated value.
Held to its full value. Also can mean a pressure accent.
A moderately sharp attack, in context of the prevailing
dynamic level.
Played very short. In modern practice also indicates martellato in string playing (heavy accent with the heel
of the bow).
Stressed and moderately short
Percussive, but with full value.