The Pipeline Variable $_

The $_ variable is simply defines as the “current value in the pipeline”. It is an alias for the $PSItem variable. If you are not yet familiar with the PowerShell Pipeline, please set aside about 30mins and go through this article: The Pipeline

'x', 'y', 'z' | % { 'current value in the pipeline {0}' -f $_}

Official name - $PSItem

Same function:

'x', 'y', 'z' | % { 'current value in the pipeline {0}' -f $PSItem}