normally you'll need three partitions for a linux install; one for swap (a few hundred megs? I'm not sure about the linuxes of these days...some ask more, some even more), one for boot (a smaller, like 16 megs or something it at least used to be) and root (where all the real stuff goes)
so mount points are as follows:
swap - no mount point needed, usually selecting "swap" as the type is just enough
boot - the mount point is /boot
root - the mount point is /
so root is just a plain "/" (without quotes) and boot is /boot got it?
swap is for swap, of course, and boot is for the bootloader that gets your system running and kernel, and root is for apps, for files, for space, for games, for everything else - unless you wish to do more partitions which is optional.
EDIT: oh yeah, about the mount point - it's like the place in your file system, where the asked partition/device is found. so when you type in a shell
(cd means change directory) it goes to your boot partition, if it's mounted. that's why it's mount point - the point, the place where a filesystem/device is mounted. and doing a
just goes to your root partition
so it's the root mount point in your filesystem. so mount point tells your filesystem, where certain devices or somethings are found, and how you - the user - can access them and where.