While modern autofocus systems have made stills work a lot easier in many ways, when it comes to video, you will often resort to different techniques, which often include manual focus. If you are new to shooting video, this great video will show you how to get proper focus using a variety of tools and techniques.
Coming to you from Saurav Sinha, this excellent video will show you how to get shots in focus when shooting video. No doubt, video autofocus has come a long way, and you can trust a lot of modern cameras in a variety of situations, but when it comes to critical shots, manual focus is often still the technique of choice for a lot of professionals. Thankfully, most modern cameras (especially mirrorless models) come with a wide variety of manual focus assistance features to make the job much easier, including technologies like viewfinder magnification, focus peaking, and more, and with those features and a bit of practice, you can become pretty proficient. Check out the video above for the full rundown from Sinha.
If you want to continue learning about shooting video, be sure to check out "Introduction to Video: A Photographer's Guide to Filmmaking With Lee Morris and Patrick Hall!"