This is a beautifully lyrical and expressive slow movement, often excerpted from larger works or performed as a standalone piece. It is characterized by its clear melodic lines and a profound sense of introspection and grace.
Achieving a truly singing tone and sensitive phrasing are the primary goals.
Adagio is rated Henle level 4 of 9 — intermediate repertoire, roughly ABRSM grades 5–6. In the RCM system it corresponds to about level 4.
You should be comfortable at Henle level 4 — meaning you can already play level 3–4 pieces cleanly. Learning it one level early is possible as a stretch piece, but more than that usually leads to months of frustration.
A typical performance of Adagio lasts about 4 minutes.
Adagio is available as free public-domain sheet music on IMSLP — the score is linked directly from this page.