An event source is an AWS service or an application created by developer that produce tasks that triggers a Lambda function to run. Some of the services that publish these events to Lambda. This is done directly by invoking the cloud function directly. E.g. Lambda can take/use records from a Kinesis stream and execute a Lambda function for every fetched message.
Many other services as CloudTrail, can act as event sources just by logging to S3 and using S3 bucket's notification to trigger AWS Lambda functions.