Not a problem if you stick to Ubuntu packages. All packages in the default apt repositories contain signed stuff, so you can install drivers (graphics, virtualbox, ...) like normal. I had it accidentally enabled when I initially installed and only noticed when I tried to build custom drivers myself.
Secure boot means only signed code can run in the kernel/ring 0. Grub, as the loader, needs to be signed as well.
Basically anything with system privileges needs to be signed.
If I remember right you need to enroll the signing key on installation, and the rest is handled automatically, but you can't use any custom kernel or kernel drivers.