Ornamentation for the flute
This part is about ornamentation in music, this means anything from notation to musical aspects of ornaments.
This definition by the old master is about complete. Thus a trill is a change between the written note and the tone, which is in the actual tonality directly above.
This means for a piece of music without any accidentals, that an a with a trill sign above, means a change to the h. But if there is a b put at the start of the line, the trill goes to the h flat instead.
If the composer wants the musician to play another upper tone, the accidental for the above note is written above the trill.