Serverless compute. You hand it code; it runs, scales and bills per millisecond — no servers, no capacity planning.
Lambda is the function-as-a-service layer underneath a large share of what runs on AWS: API backends, event pipelines, cron, glue between services. Code goes in, AWS decides where and how many copies run.
It ships a managed runtime for each supported language, patched and released by AWS rather than the customer, and it exists in every commercial region — which is what makes a change to it a global-blast-radius change.