Magisk is an app which helps users to root their phone. With the help of Magisk you can run banking apps and also pass SafetyNet tests.
Magisk is now available as an alpha version, which means that it's not fully stable yet. If something goes wrong, re-flashing magisk with TWRP or Fastboot might fix your problem. However, we expect magisk to be more stable in future updates.
Since magisk is installed separately from the existing system, you don't have to worry about your warranty getting void by flashing magisk onto your device. magisk runs through a ramdisk and doesn't modify the system partition unless you let it do so (e.g: magisk phy is used to patch the system partition). magisk is even compatible with other modified boot images.
Google has caught magisk, however magisk has always been open about its existence on XDA. magisk doesn't break SafetyNet for Pokemon Go or any games which are hard coded to detect if your phone is rooted. magisk developers are working towards it, however don't expect everything to work perfectly until magisk gets stable in future updates.
Facing an issue with magisk, here you can check what's the main issue while installing magisk.
Is It's safe to use Magisk?
It is the nature of all mods, which are designed to override or ignore parts of the system you have on your phone, that they have the potential to mess with various things in an unexpected way. For this reason it's always recommended that you do not leave Magisk enabled 24x7 under any circumstance - rather only boot into it when required for whatever purpose. Additionally, there are situation-dependent precautions that everyone needs to take before running them live on their device.
Update Regarding Safety Concerns
People using magisk can rest assured knowing they are being tracked by people who truly care about their security and don't regret installing it.
Why should you use magisk ?
Magisk is a project initially born as a low-level toolset and grew into much more than that. It wants to offer you maximum flexibility, no locked-in decisions for installation, and make your device yours again.
It means you can do anything with your device--partner it with one or multiple operating systems (Nougat or Oreo? OnePlus One or OnePlus 5T?), install tweaks, develop launchers (or commit any other modification), and importantly do all of this without breaking SafetyNet.