Understanding the options in the GRUB menu

The version numbers refer to the kernels you have installed. You can get the version number of the running kernel like this:

$ uname -r
4.14.0-rc4-sound-23

You can see all kernels installed by looking in /boot:

$ ls /boot/vmlinuz*
/boot/vmlinuz-4.13.0-19-generic
/boot/vmlinuz-4.13.0-19-generic.efi.signed
/boot/vmlinuz-4.13.0-21-generic
/boot/vmlinuz-4.13.0-21-generic.efi.signed
/boot/vmlinuz-4.14.0-rc4-sound-23

_{Note that the GRUB menu is populated by GRUB's configuration file, which is written to when update-grub is called. This is done automatically when you install or remove a kernel package (or run a system upgrade that includes one), but if you change GRUB configuration in /etc/default/grub, install a kernel you have compiled or downloaded from outside your package manager, you need to run sudo update-grub to see changes in the GRUB menu.}

The kernels are listed in the GRUB menu with recovery options (mainly for fixing your system when it is broken) and in your case with an option to use Upstart, the older init system used by Ubuntu before systemd became the default init system in 15.04.

Possibly relevant: Remove upstart option from GRUB

These options in the GRUB menu (usually only visible under Advanced Options) to boot different kernels come in useful if you have problems; for example, a newly installed kernel is buggy on your hardware, and you need to boot an older one to get the system working well enough to start debugging.