Android System WebView is, in Google's own words, a system app that uses Chrome technology to display web content on other Android apps. This app is installed by default on all Android devices. In principle, it has to be kept updated to ensure its integrity and that it works correctly with other apps and games.
Usually, you don't have to open Android System WebView or do anything with it. After all, it's a system app, so it runs in the background, doing what it's meant to do without you noticing. Its purpose is simple, but very important: with it, web content is displayed correctly on other apps and games. What does that mean? Basically, it means that Android System WebView intervenes every time you open a website within another app, whether it's Facebook, Twitter, LinkedIn, or a video game.
It's important to keep in mind that from Android 7 onwards, most smartphones rely on the Google Chrome app itself to do what Android System WebView used to, so it's possible that on many newer devices, it's already disabled.
Android System WebView is a particularly important system app for devices running Android 7 Nougat or lower. On newer devices with Android 8 or higher, it's not necessary to have this app, and although it's still updated, it's usually disabled by default to avoid interference with Google Chrome.