You need to set an S3 event trigger on your Lambda to respond when data is added to your bucket from another S3 bucket. Which event type do you configure?

AWS Lambda is an event-driven, serverless computing platform provided by Amazon as a part of Amazon Web Services. It is designed to enable developers to run code without provisioning or managing servers. It executes code in response to events and automatically manages the computing resources required by that code.

Developers can trigger Lambda from over 200 AWS services and software as a service (SaaS) applications, and only pay for what they use.