I'm trying to install Ubuntu (16.04) on VirtualBox, now I'm getting the option Erase disk and install Ubuntu. I have another Ubuntu (18.04) on my VirtualBox with some important things on it. Will the option above erase my Ubuntu 18.04? Thanks
It will not if you are trying to install Ubuntu 16.4 on an other vbox disk.
If you click on "new" (the blue button at the top of the virtual box application) and then follow the instructions there, you create a new file on your system that behaves like a hard disk for virtualbox.
So if you just had your Ubuntu 18.04 image on the left pane you will now see a second image with the name that you specify after clicking on "new".
The images are seperated from each other and your install on the second (16.4) image wont touch your first image.
If you "insert" the 16.4 iso to your 18.4 machine then it will erase the data.
Here are detailed instructions and you will see that the person in the video has more than one image on her virtualbox installed: https://www.youtube.com/watch?v=diIW3fgewhI