Track0 (pronounced 'track zero') is the area located at the start of a computer disk and is predominantly used to store information regarding the layout of the disk (the partition table) and executable code needed to boot an operating system.
Near the end of the MBR is the Partition Table: a predefined structure containing the layout of the disk.
Track0 is also increasingly used to store licensing information for particular pieces of software because it is an area of the disk outside of the OS filesystem and not seen or used by most computer users (and therefore cannot be easily modified or replicated).
As with many (now legacy) original PC specifications, the terminology stuck and the first 63 sectors of modern hard disks are still referred to as Track0.
With DOS, the first track of a floppy disk is used to store system information.