Anyone familiar with custom ROMs will more than likely have used TWRP on several occasions. TWRP (Team Win Recovery Project) is a custom, community-driven recovery that allows you to root your device, flash third-party ROMs and a whole lot more. Very often, it is required as the stock recovery that comes with a device is highly limited in terms of what it can do and discourages users from tinkering around with their devices too much. To be fair, you don't really need to root your device anymore, unless you have a really specific reason for it.
Despite the open-source nature of the Android operating system, companies don't want users to mess around with their devices a whole lot and have put safeguards in place that can detect any unauthorized modifications. Samsung implemented a hardware switch that trips when you flash a custom recovery, effectively rendering Knox useless and voiding your warranty. The process cannot be reversed even if you were to undo all the changes you made to your device and you won't be able to use features such as Samsung Pay and Secure Folder anymore.
The Exynos variants of the Samsung Galaxy S9 and S9 Plus have unlockable bootloaders, which makes the process of flashing TWRP a whole lot easier. The same can't be said for the Snapdragon versions, though. Several Snapdragon-powered devices have locked bootloaders, which can be a pain to get around. However, thanks to our friends over at XDA Developers, an unofficial version of TWRP is now available for Snapdragon versions of both the Galaxy S9 and Galaxy S9+. Users in Hong Kong and several Latin American countries that use Snapdragon-powered Galaxy S9 and S9+ can now flash TWRP on their devices. Unfortunately, users in the US will have to wait a bit longer, as there isn't a method to unlock the bootloader on the US variants yet.
Once again, we wouldn't recommend inexperienced users to try this as there is a high likelihood of your shiny new S9 being rendered useless if you don't know what you're doing. Here are the links for the Galaxy S9 and Galaxy S9+. There aren't too many custom ROMs going around for the two devices yet, so you might want to hold off until that happens.
News Source: XDA developers