Lambda reads records from a stream at a fixed cadence (e.g. once per second for Kinesis data streams) and invokes a function with a batch of records. Batch Window allows you to wait as long as 300s to build a batch before invoking a function. Now, a function is invoked when one of the following conditions is met: the payload size reaches 6MB, the Batch Window reaches its maximum value, or the Batch Size reaches its maximum value. With Batch Window, you can increase the average number of records passed to the function with each invocation. This is helpful when you want to reduce the number of invocations and optimize cost.