This can be used to give your webcam a round border for example: The Image Mask/Blend filter gives us the option to use the Color or Alpha Channel of an Image as a Mask or to Blend an Image (multiply, addition, subtraction) over your Scene or Source. But let me explain what the different filters allow you to do. You can add them by right-click your desired Scene, Source or Device and selecting 'Filters' (for Audio Devices, click on the gear icon next to your device). The following filters are available in OBS Studio 23.0.0:

In OBS Studio we have the ability to add filters to our Sources, Scenes and even our Audio Devices.