Lambda Function Handler is a function in your code, that AWS Lambda can invoke when the service executes your code.

def handler_name(event, context): 
    return some_value

event – AWS Lambda uses this parameter to pass in event data to the handler. This parameter is usually of the Python dict type.
context – AWS Lambda uses this parameter to provide runtime information to your handler. This parameter is of the LambdaContext type.
Context Methods-
get_remaining_time_in_millis – Returns the number of milliseconds left before the execution times out. Context Properties-
function_name – The name of the Lambda function.
function_version – The version of the function.
invoked_function_arn – The Amazon Resource Name (ARN) used to invoke the function. Indicates if the invoker specified a version number or alias.
memory_limit_in_mb – The amount of memory configured on the function.
aws_request_id – The identifier of the invocation request.
log_group_name – The log group for the function.
log_stream_name – The log stream for the function instance.

Invocation Types:
AWS Lambda supports synchronous and asynchronous invocation of a Lambda function.