A window is a time in which deployments are allowed to a particular environment. Deployments occurring within the window are allowed to execute immediately (provided no other approval is configured), whereas, deployments outside of the window require the approval of a configured approver if the request is to be an exception to the window. Deployments outside of a window will be scheduled for the start of the next available window.
For details on Cron Expression Syntax see
A window is defined by a cron expression that resolves to any number of one-second durations. To create a 1-hour window, an expression like * * hour ? * * is used. To create a 1 second window, an expression like 0 0 hour ? * * is used. Projects will deploy at the next available window/second unless a delayed start is given. The following picture shows a window for the "Development Environment" that starts at 1pm every day and goes until 2pm. When a deploy is requested before 1pm, it would schedule for 1pm. If between 1pm and 2pm, it will run immediately. After 2, it will wait until the next day at 1. If exception to Window is selected, then the Approvers (Selected Groups) need to approve the deployment which will run either immediately, or at the scheduled time if one was given.