ramfb is a very simple framebuffer display device. It is intended to be configured by the firmware and used as boot framebuffer, until the guest OS loads a real GPU driver.
The framebuffer memory is allocated from guest RAM and initialized using the firmware config interface (fw_cfg). edk2 (uefi firmware) has ramfb support. There also is a vgabios, which emulates vga text mode and renders it to the framebuffer.
Most interesting use case for this is boot display support for vgpus.
vfio has a not hot-pluggable variant, which allows to enable ramfb
qemu -device vfio-pci-nohotplug,ramfb=on,...
Once the guest OS has initialized the vgpu qemu will show the vgpu
display. Otherwise the ramfb framebuffer is used. The firmware
messages, boot loader menu and efifb/vesafb output all will show
There also is a standalone device, mostly intended for testing:
qemu -vga none -device ramfb.
Even though it is possible to use ramfb as primary display it isn't a good idea to actually do that as this isn't very efficient.