VRAM Debugger asset is used to provide information about number of loaded (Unpacked) textures into GPU memory as well how many size these textures take.
Each unpacked texture page take MB into VRAM, once number of texture pages exceeds available VRAM your game theoretically crashes. There is a lot of difference how different GPU and drivers handle this situation and you newer known, what HW your audience use it, especially on Windows. For that reason this tool help you manage and debug currently VRAM usage of your project. For example 25 unpacked texture pages with resolution 2048x2048 take 400 MB into VRAM. If you want to support 512MB GPU you shouldn't exceed these number, because you also need some free space for other system stuff.
Usage is very simple and you need to setup only few things to debugger overlay works correctly:
Only basic knowledge of GML is need for implementation, however overlay is more usable on more complex projects, where you plan to manage texture groups.