What to erase when returning to factory settings?

I bought a computer and uploaded my time machine onto it. Turns out that in doing so, I transferred problems from my old computer to my new. So, I want to return the new computer to "factory settings" to get rid of anything I brought to the new computer.

Following Apple Support instructions, I restarted in Recovery Mode and opened Disk Utility. The instructions say, "From the sidebar in Disk Utility, select the disk or volume to erase.

For most reasons to erase, you should erase the disk, which also erases all volumes on that disk."

My question: I don't know what to erase. is it Macintosh HD, or something above (the disk, or the top level name of the computer itself)? Thanks for the help!