I'm relatively new to teaching keyboard, and still getting to grips with some of the terminology.

Reading the introduction for a piece in Electronic Keyboard Collection Book 1 (Trinity Guildhall) p 16 it says
"The use of harmony control would be useful here..."

What do they mean by harmony control, and how do I do it?

Most keyboards have a harmony button which when pressed will add extra notes to the melody line. You can usually alter what exactly happens by accessing further buttons.The choices are usually to add a second part (probably in thirds), add 3 notes, add block chords, make the notes trill . Other effects are playing in octaves and tremolandos (this effect can be useful on a xylophone as it will sound if a pair of beaters is hitting the notes in rapid succession), or even echoes.

The effects that are available tend to vary according to whatever voice is selected and the effect will only work when the styles are in use.

The overall effect is to make the player sound better than they are!!

Look for the button and try them out.

Thanks!