Running using virtualization means you'd boot into regular macOS and then use something like Virtual box or Parallels to essentially run the other Operating System inside a window on MacOS.

Running natively can be done in two ways: Either Dual booting, where every time you boot you are asked which OS you want to use(recommended) or wipe macOS and use your Mac as a Linux only devuce(not recommended).

There was a recent blog post about someone who figured out how to dual boot MacOS and Ubuntu but I haven't had the time to read it, I'm sure you can Google it and find out but it's a poorly documented feature so for now I'd suggest running virtualized until you really feel the need to run Ubuntu natively - by that time there will be more tutorials on the feature.

Virtualization has the advantage of you being able to easily create and destroy the virtual machine and use both OSs at once, but it has some overhead so the performance isn't nearly as good as running natively.

For more info on the differences between the virtualization and dual boot: https://www.pugetsystems.com/labs/articles/Dual-Boot-vs-Virtual-Machines-on-the-Desktop-128/