If you are running Windows and would like to install Linux on your system, you must to free up some space on your hard drive to contain Linux. Changing the layout of your hard drive is called partitioning.
The Ubuntu installer's partitioner is one of the safest ways to partition a hard disk. However, do not use this as an excuse to not back up your important files. Always perform a backup before any partitioning operation.
There are several ways to partition a hard drive for Ubuntu. If you have data on the hard drive in partitions with free space left on them you can resize those partitions to create space for a new partition. This enables you to keep your existing data and operating system so you can install Ubuntu alongside it. If you want to dual boot Ubuntu and Windows or another Linux distribution this is what you want.
You can also delete existing partitions and create new partitions in the free space for Ubuntu. This will cause you to lose any existing data but you will have more room for Ubuntu as it is the only partition on the drive.