Complex sentence: Until the 1930s, due to lack of amplification which made it difficult to be heard over drums, piano or horn sections, jazz guitarists tended to act as accompanists, strumming chords as part of the rhythm section.
Split into simpler sentences: Until the 1930s, before guitar amplifiers were widely used, it was difficult for jazz guitarists playing acoustic instruments to be heard over drums, piano or horn sections. As a result, jazz guitarists tended to act as accompanists, strumming chords as part of the rhythm section.