They are called “fiorituras”, and they are a kind of ornament (literally: “Flourishes”). You can find a similar one in the left hand of Chopin’s Etude in C#m op. 25 no. 7 bar 27, or in Chopin’s Nocturne in C#m (Op. Posth.), right hand, bars 58 – 59.<snip>
Side note: what piece is this?
I'm not surprised that fiorituras are not mentioned in a theory book. They're a technique-related thing, really. Most theory books tend to focus on pure theory and analysis.