think of a hard drive as a plot of land. You could issue a building permit for the whole thing--or you could subdivide it an issue permits for different usages on each parcel.
The default when installing Windows is usually one partition filling the whole disk.
For Linux, you will make a minimum of 2 partitions: one for the root directory, and one for swap space. The Linux installer will normally do the partitioning, so you would not need to be concerned about it. Once linux is running, you will find powerful tools for viewing and modifying partitions.
When you install, leave some empty space on the disk. Linux will easily install in 15Gb or so---leave the rest unpartitioned for later use.