An example of cue notes. This example from the 2nd horn part Overture to Der Freischütz contains cue notes showing the 4th and 3rd horn parts, in order to aid proper entrance. Play (help·info)
A musical cue is where another instrument's music is shown on a piece of music. It is up to the performer or the conductor on whether the section should play it. A cue may also function as a guideline for another instrument for improvisation or if there are many bars rest to help the performer find where to come in.
The cued instrument is indicated and notes smaller than the rest.