This screen appears if "manual" partitioning was chosen on the "Disk Configuration" screen.
On this screen is a list of partitions currently on disk and this menu (a very simple Linux install used as an example):
Here is a list of the current disk partitions:
* hda
\-> hda1
\-> hda2
\-> hda3
Do you wish to modify the above list using 'fdisk'?
1. yes
2. no
>
If you select "1" or "y" you will ask to specify which disk you wish to partition. You can designate a disk by its /dev/ directory filename (hda, sda, or what have you). Once you have entered a disk the installer sends you into 'fdisk' as if you had typed:
fdisk /dev/[what-you-entered]
at a shell prompt. If you need help with fdisk please look at the man page for fdisk or places on the web like:
http://www.doc.ic.ac.uk/lab/labman/lookup-man.cgi?fdisk(8)
After you have modified the disk partitions to your satisfaction write them to disk and select "0" or "n" when asked to repartition to go on with the installation.