Sure, the Get-HardDisk cmdlet returns one or more objects and places these objects in the pipeline (the '|' operator).
The next cmdlet, the Where-Object cmdlet, receives the objects one-by-one.
The object is available to the Where-Object cmdlet as the $_ variable.
Objects have properties, in other words values, associated with them.
To retrieve a value for a specific property one uses the dot-notation.
In this case $_.Name means the Name property of the object that arrived through the pipe.
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