What aspect of music does 'tempo rubato' primarily focus on?

'Tempo rubato' primarily refers to the flexible approach to rhythm within a musical piece. It allows performers to manipulate the tempo by speeding up and slowing down the pace for expressive purposes, rather than strictly adhering to a fixed meter. This technique enables musicians to convey emotion and nuance, enhancing the interpretation of the music. It is especially prevalent in romantic-era music, where expressive timing can significantly impact the overall performance. The other aspects, such as harmony, melody construction, and scale variation, do not encompass the rhythmic flexibility that is central to the concept of 'tempo rubato.'