Artificial lighting can be a lot to try to learn when you are new to the subject, with a wide variety of tools, setups, rules, and techniques to get down. One of the most useful tools is the grid, and this excellent video will introduce you to how they work and when you should consider using them for your own work.
Coming to you from our good friend, Pye Jirsa, with Adorama TV, this helpful video will introduce you to grids and give you some examples of their usage. The need for grids arises from the fact that a lot of modifiers, especially large options used for providing softer light, tend to spill light all over. That is not necessarily a bad thing, but often, you might not want light splashing on the backdrop and the rest of the scene instead of just the subject. This is where a grid comes in handy. It restricts the light by narrowing its spread off the axis of its output, with its level of control typically specified by the angle of its output (e.g. a 20-degree grid). You will often see grids used with softboxes or beauty dishes to precisely control the light, thereby increasing the drama of the scene or allowing the photographer to light the rest of it as they see fit. Check out the video above for the full rundown from Jirsa.