Ubuntu provides a PartitioningBasics wiki page that provides additional details relevant to the Prepare partitions page shown here.
If you plan on having multiple Linux-based operating systems installed you may want to experiment with modifying the boot loader setting. Installing the boot loader to the same partition as the root partition instead of the drive will ensure that all drivers that were added to the initramfs are installed during boot-up.
The drop-down bar indicates where the boot loader will be installed. If GRUB is going to be used, the first Linux-based installation needs to install the boot loader to the MBR, which is accomplished by selecting the device, not the root partition. Additional Linux-based installations should be installed to their root partition. The example shown here has a Centrych 64 bit OS installed to