A windowed mode for a software application is a mode in which the program appears in a smaller window rather than on the device's full screen. This display mode is a typical part of many applications made for Microsoft Windows and other similar OS designs.
Techopedia explains Windowed Mode
There are many different ways to make a program operate in windowed mode. For example, in Microsoft Visual Basic and similar languages, a developer can use a resource called DirectX, a collection of application programming interfaces (APIs), to implement a windowed mode for an application. There are also many different kinds of programs, including games and virtualized applications, that can benefit from running in a windowed mode. The main benefit is usually the end user’s ability to monitor multiple programs on a screen at once.
Some problems related to a windowed mode revolve around graphical displays. For developers who want to create a windowed mode for a program, understanding its effects on rendered graphics within the window is ultimately important. In addition to a loss of graphic quality, some programs can have issues with accessing memory to render the graphics correctly. All of this contributes to a common dialog between developers about how best to implement a windowed mode in various scenarios.