Lightstep is a distributed tracing platform for debugging microservices. When a user request flows through 5+ services, where does it slow down? Lightstep collects a "trace" (path of request through services), shows timing per service, highlights bottlenecks. Developers fix the slowest service and move on. Without tracing, debugging microservices is blind (logs are fragmented, no request timeline). With tracing, it's surgical (clear bottleneck, clear fix).