Backline is the set of instruments and amplification placed on stage so the band or artist can perform: drums, guitar and bass amps, keyboards and percussion. Personal instruments such as guitars and basses are normally brought by each musician.
What it usually includes
A drum kit with its hardware, guitar and bass amplifiers, keyboards or stage pianos, percussion and, depending on the show, wireless systems and in-ear monitoring. The exact list depends on each artist's technical rider.
When renting makes sense
Renting backline avoids moving heavy gear between cities, reduces the risk of transport damage and lets you request the exact model the artist needs. It is especially useful for festivals, tours and events with short load-in times.