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Disk Crash - What exact does it mean ?

Conventional hard drives (not the  the modern SSD flash drives) contains moving parts so will eventually suffer mechanical failures., leading to infamous "head crash", frequently the  kiss of death  to your precious data.
Diagram on components of hard disk
Spinde Axis
Data Platter(s)
Read Write Head
Actuator Arm
Circuit connecting to external PCB
With reference to the above diagram, you can see that the disk contains one or more spinning data platters rotating about the spindle axis at extremely high speed, generally between 5400 revoluton per minute  (rpm) to 7200 rpm or even more.  The data platters are the place where your data is stored as magnetic bits and bytes. The greater the capacity of the hard disk, the higher the  magnetic density (and  therefore harder to reccover when troubles emerge).
The actuator is made up of a wired coil placed between two strong magnets. When current is passed through, the coil, which  is part of the actuator assembly, will move with it together with the actuator arm about the actuator axis. Reversing the current will move the actuator arm in opposite direction.  At the  tips of the actuator arm  is the  sensitive and tiny read write head (hardly seen by naked eyes) where it floats on a cushion of air that  is well below 5 nanometers, 2000 times thinner than a strand of hair ! With the movement of read write heads back and forth above the rotating platter, data bits are read or written through out the platters.
For some reasons such as impact, dust contamination, malfunction and the like, the write head instead of floating above the  data platters, will come into direct contact with the  platters at high whirling speed, scraping with  it the platters' magnetic coating, which  is also your data.
Read write head crash
Bad head touching or crashing on the  platter
Platter damaged by crashing  head and  seen as circular scratch ring