HDD, for "Hard Disk Drive", designates a hard disk. It is a magnetic mass memory used to store data.
An HDD contains aluminum platters covered with a magnetic layer where data is stored in binary code.
The writing and reading of data is done via one (or more) hard disk heads. According to the electric current which is subjected to the head, during writing, this one modifies the magnetic field of a zone of the platter to write the data there in binary. During playback, the magnetic fields of the different zones of the platter are at the origin of an electric current which is transmitted to the terminals of the head.
The voltage of this current makes it possible to reconstitute the binary code of the recorded data.