"In layman's terms, partitioning a hard drive can make it behave like multiple, independent non-partitioned hard drives for most practical purposes; the main difference with separate hard disks is that partitions of the same hard disk share their hardware (and therefore hardware failures) and it is possible to resize existing partitions." -Wikipedia

A partition is like a dividing wall on your computers hard drive which allows separate the space on your hard drive for different things. Most commonly seen on a windows system as your main drive and backup drive, on a Linux system it is a bit different you have a / (root) partition where all your system and program files go, /home where all your personal files go (seen as /home/username, /swap which acts like more RAM for your computer.