Here is the note on this lesson from Duolingo desktop version:
Ter- prefix The ter- prefix marks a passive verb, almost like the di- prefix. However, ter- is different in that it is used in situations where the verb is done spontaneously (Dia tersenyum), unexpectedly (Adik terjatuh), or denoting the state of an object (Suratnya sudah terkirim). Example: Botol ini diisi dengan teh [oleh Tini) = This bottle is filled with tea (by Tini). Botol ini terisi teh = This bottle is filled with tea.