A polyphonic instrument, such as an organ or piano, allows several notes to be played simultaneously. Brass or reed instruments are monophonic, which means that only one note can be played at a time. EXS24 mkII lets you choose an appropriate keyboard mode for the type of instrument that is loaded. You are free to use a monophonic mode for polyphonic instruments, which allows playing styles that are not possible with polyphonic instruments.
Click the Legato, Mono, or Poly button.

In Legato mode, only one note can be played at a time. The envelope generators are retriggered only if you play staccato—releasing each key before playing a new key.
In Mono mode, staccato playing retriggers the envelope generators every time a new note is played. If you play in a legato style (play a new key while holding another), the envelope generators are triggered only for the first note you play legato, then they continue their curve until you release the last legato played key.
In Poly mode, several notes can be played simultaneously.
Note: In many classic monophonic analog synthesizers, the behavior in Legato mode is referred to as single trigger, while Mono mode is referred to as multi trigger.