What is a Partition
A partition is just an entry in a partition table, it has a start location and end location.
The partition table contains the locations of the partitions on the disk, which contain filesystems.
The entry just says the partition starts here and ends somewhere else.
So in that space a filesystem is located.
Usually, there is a partition table at the start of the disk, and it has the locations of all the partitions on the disk.
That way, a disk can be split into several pieces (partitions), each piece containing a filesystem.
A partition can be larger than the filesystem it contains, and the filesystem can be resized to make use of the full space of the partition.
On the other hand, if someone wants to shrink a partition, first they must resize the filesystem to be smaller, then change how big the partition is in the partition table.
If the partition is deleted from the partition table, the filesystem remains intact and all the files and data is fine.
If we know the location of the filesystem, we can manually access it without using the partition and partition table. Also if we want we can restore the entry in the partition table.
Depending on the partition table, there is some limitations, for instance msdos partition tables can only have 2TB partitions and less in size.