。
She wrote something in her notebook.
Literal
She [topic-は] notebook [in-に] something [object-を] wrote-in.
書き込む is a compound verb 書く ('write') + 込む ('go into, put into'), giving 'write into / fill in / write down (in a designated place).' The ~込む adds a sense of inward direction — entering, fitting into, filling. Without 込む, just 書く + に would also work but feel less specifically 'jot into a particular spot.' 何か ('something') here is the indefinite — paired with を, it's the object of writing. The polite 書き込みました (masu past) gives the sentence a neutral, narrating register — typical of textbook-style Japanese prose.