A partition is a dedicated space on a disk. In windows there's not much use to partition a disk, but it can be done, and you would have a C: and D: drives with only one physical disk. I used to do that back in the day to have a partition for backups.

If you only have one disk and want to have multiple OS, you need to partition the disk, so that each OS can write their data without interfering with one another. Essentially what you're doing is, like you said, putting a wall between areas in the disk, but you can do that regardless of having different OS in each side.

In Linux things are a bit different, the representation of your disks is a file inside /dev, for example the first disk (non-nvme) Linux finds will be /dev/sda, the next one will be /dev/sdb so on and so forth, but since disks can be partitioned the first partition in your first disk is /dev/sda1, then /dev/sda2, etc. Then there's a file called /etc/fstab that has lines like /dev/sdb3 /home, this means that the 3d partition in the second disk will be accessible in the folder /home. You don't really need to worry about this file in general, during the installation there will be a nice GUI to let you say which partition goes where.

How is that useful? Well, if you have the system in /dev/sda2 and your /home folder in /dev/sda3 you can format /dev/sda2 and reinstall the system or change the distro entirely without losing your data stored in /home.

PS: I'm simplifying some stuff, but for reference :

• you might see partitions jump from 2 to 6 in older systems, this is due to limitation in partitioning schemes for old disks
• if you have a really old computer you will see /dev/hda1, this is because the s in sda refers to SATA, which essentially all disks are nowadays
• nvme drives are /dev/nvme0n1
• /etc/fstab has other parameters to tell it certain flags like mount read-only. Also it rarely used /dev/sda1 style naming because that might change if you swap the cables in your computer, instead it uses a unique identifier that's points to the correct partition regardless of order.
• Partitions are not really a wall, instead the first bytes of a disk contain a table saying stuff like byte 0-61648716832 partition 1, bytes 61648716833-9274816418393 partition 2, etc. Old drives had limited space in that table so you had to create one partition for the rest of stuff and repartition that again, which is why partition numbers jumped from 2 to 6.

but all that's besides the point.