Plex is media-playing software that can stream a file from one device to another over your local network. The base product is available for free on Windows without ads. Or you can subscribe to the Plex Pass, which adds features such as DVR recording, syncing files to mobile devices, and coordinating with cloud storage services to stream media from the cloud to your device.
The base version is very capable: If you just want to stream a video from your computer to your TV or a mobile device, the free version of Plex doesn't put up many roadblocks. It will encourage you to create a Plex account, but you can ignore that and just tell it to scan your PC for media to share locally. You can also create and manually add other folders, and Plex will add integrate that media and track those folders for future changes.
Plex is everywhere: You need a Plex app on both ends of your stream, so it needs to be available on a variety of platforms for everything to go smoothly. Thankfully, it's one of the most widely distributed apps on the planet: You can get it for Windows, MacOS, iOS, Android, Linux, game consoles, Apple TV, Roku, and Chromecast. It's not free on every one of these platforms, but the price tag is less than you'd pay for a deli sandwich and helps to fund future development.
Easily scales to your environment: If you're trying to stream media over a spotty Wi-Fi connection, you can tell Plex to reduce the quality level, which reduces the network speed that you need for smooth playback. Or, if you have a fast and reliable connection, you can scale back those network optimizations and keep the video looking as good as possible.
Adding media to your library can take a while: Your media library is easier to visually scan if you're using cover art instead of navigating by file name. Plex can also provide the year of release, the names of actors and directors, and other parameters for sorting and searching more easily. The flip side is that this added functionality requires processing time. If you have a sizeable amount of MP3s and whatnot, be prepared to do something else to occupy your time until Plex is ready to stream it all.
While you can use VLC media player (Windows, Mac, Android, iOS) to stream media over a local network, Plex provides a much friendlier way to do that. And it has a proper media collection interface and better tools for handling different network speeds.