A flash grid is a simple light modifier that limits the direction of the light. The effect that it produces is similar to a snoot modifier and a vignette filter.
There is a wide range of girds on the market that answer to specific needs. I opted to design and print my own. The advantage of making your own grid is that you can change it to suit your needs. In this instance I prepared two, a short one and a long one. The grid goes over a speedlight flash unit.
Here you can see the striking differences. The narrow grid illuminates just the center of the scene, the wide grid produces a gradual light drop off, and the last image is a reference without a grid.
You can download and make your own. Hopefully my design will fit your speedlight unit, otherwise you may want to play around with the Blender file.