Mozart's '6 Tonstücke' are likely referring to individual movements or smaller, less famous works from his output, rather than a distinct collection with this title. If referring to movements within larger sonatas or other works, they exhibit his hallmark clarity, balanced phrasing, and often graceful melodic lines.
Emphasize clarity of articulation, especially in Alberti bass figures, and nuanced dynamic shaping.
6 Tonstücke is rated Henle level 5 of 9 — late intermediate repertoire, roughly ABRSM grades 6–7. In the RCM system it corresponds to about level 6.
You should be comfortable at Henle level 5 — meaning you can already play level 4–5 pieces cleanly. Learning it one level early is possible as a stretch piece, but more than that usually leads to months of frustration.
6 Tonstücke is available as free public-domain sheet music on IMSLP — the score is linked directly from this page.