Angular Client allows the user to add and delete images from S3. It also calls an API Gateway endpoint to retrieve details for all images uploaded.

Save/Delete Lambda Function will handle image upload and delete events from S3. It will be invoked by S3 when a new image is uploaded - the function will use the supplied event data to call back to S3 and retrieve the image. The image details will be saved to DynamoDB. As an image is deleted the function will use the supplied event data to identify the deleted image and remove the associated details from DynamoDB.

Retrieve Image Details: Lambda Function will retrieve image details from DynamoDB and return JSON results.

Dynamo DB: a single Dynamo DB table will be used to persist image details. All interactions with DynamoDB will happen via the Lambda functions.

API Gateway: an API Gateway endpoint will be used as a bridge to the Retrieve Image Details and Lambda function from the web app.

That should give you a good overview of how Lambda functions work. Also, note that some cloud providers call this “Function as a Service” FaaS.