Tertia means harmonising a melodic line by adding parallel 3rds to it.

Mozart-Allegro D major

Composition with extensive use of Tertia Quiescenza, that is direct motion 3rds to a melodic bass with a Quiescenza voice (pedal tone) on tonic or/and dominant added. Mozart embellishes this voice with an acciacatura.
Also check ‘Mama’ by P.I. Tschaikowsky. Same technique. When ascending the scale with Tertia Quiescenza, a progression from Tritto to Finale is almost automatically generated (when descending: Reverence followed by Prinner