A grace note is subsiddary note you play just before a main note.
Typically a grace note is one scale tone above or below the main note.
If the main note is a C, for example, you could lpay either a B or a D as the grace notes. There is no hard and fast rule; grace notes can be any note within the underlying scale.
You add a grace note to lead into and emphasize a key tone in the melody.
The grace note is typically played right up against the main note, not precisely in rhythm, which is why it isn’t notated as a normal sixteenth note.
It’s almost like a little flip just before the main note.
A grace note can be a singlenote, or you can play multiple grace notes, typically in a run up to or down to the main note.
For eg. when you lay a turn that starts before the beat, the notes of the turn are grace notes.
Just make sure that the grace notes are played before the expected beat, and that they don’t delay the main melody note.