Appoggiaturas are accented non-chord notes which move by step (up or down) to a chord note to resolve.
In this example, the chord is C major. The note D is an appoggiatura. It does not belong to the chord, so it is a non-chord note. It falls on the beat (not between beats), so it is accented. It moves by step to the chord note C, which is its resolution.
![appoggiatura example](https://mymusictheory.com/wp-content/uploads/harmony/decoration/appoggiatura-example.png)
In early music, appoggiaturas were normally written as small-sized notes, to make it clear that the notes are decorative and not part of the harmony. By the Classical era this practice was dying out, but the appoggiatura as a decorative figure was (and is) still widely used in its written-out form (in normal sized notes).
Appoggiaturas are mostly found in the melody part, or in a soprano line.
In this tune, which is in G major, the E, A and C are best described as appoggiaturas, each resolving downwards to a chord note. This allows the harmony to move along with one chord per bar. The only alternative would be to use a much larger number of chords, which would sound very hectic.
![melody with appoggiaturas](https://mymusictheory.com/wp-content/uploads/harmony/decoration/melody-with-implied-appoggiaturas.png)
Appoggiaturas are often approached by a leap, as the E and C are in this example.