A grace note is a note usually displayed as a smaller sized note which does not have duration of its own. The duration of gracings is borrowed from the next normal-sized note (to which it usually connected with a slur). Grace notes are inserted with the grace note menu
It is possible to cluster two or more grace-notes to form double or triple gracing.
Although grace notes are of the same size as cue notes, they serve different goals, and are interpreted very differently.
How grace notes are played depends on settings in the playback dialog.