Multi-part payments (MPP), also referred to as multi-path payments, are a feature initially added to the Lightning Specification in BOLT #4. MPP allows the sender to split a payment across an arbitrary number of channels and routes, combining liquidity in all channels up to the desired amount.
MPP dramatically improves the resiliency and connectivity of Lightning by splitting large payments into smaller parts, making it easier to find routes. Each payment part can take a different path, avoiding bottlenecks and channels with insufficient liquidity and making the payment process more resilient to individual channel failures.