EFI/ubuntu/grubx64.efi on the EFI System Partition (ESP) is the GRUB binary, and
EFI/ubuntu/shimx64.efi is the binary for shim. The latter is a relatively simple program that provides a way to boot on a computer with Secure Boot active. On such a computer, an unsigned version of GRUB won't launch, and signing GRUB with Microsoft's keys is impossible, so shim bridges the gap and adds its own security tools that parallel those of Secure Boot. In practice, shim registers itself with the firmware and then launches a program called
grubx64.efi in the directory from which it was launched, so on a computer without Secure Boot (such as a Mac), launching
shimx64.efi is just like launching
grubx64.efi. On a computer with Secure Boot active, launching
shimx64.efi should result in GRUB starting up, whereas launching
grubx64.efi directly probably won't work.
Note that there's some ambiguity possible. In particular, if you want to use a boot manager or boot loader other than GRUB in a Secure Boot environment with shim, you must call that program
grubx64.efi, even though it's not GRUB. Thus, if you were to install rEFInd on a Secure Boot-enabled computer,
grubx64.efi could be the rEFInd binary. This binary would probably not reside in
EFI/ubuntu, though; both it and a shim binary would probably go in
EFI/refind. Also, as you've got a Mac (which doesn't support Secure Boot), there's no need to install rEFInd in this way; it makes much more sense to install rEFInd as
EFI/refind/refind_x64.efi (its default location and name).
Note that the rEFInd documentation includes a whole page on Secure Boot. Chances are you won't benefit from reading it, user190735, since you're using a Mac. I mention it only in case some other reader comes along who's trying to use rEFInd in conjunction with Secure Boot.