What does "%" (percent) do in PowerShell?

Question

When used in the context of a cmdlet (such as your example), it's an alias for ForEach-Object:

> Get-Alias -Definition ForEach-Object

CommandType     Name                                                Definition
-----------     ----                                                ----------
Alias           %                                                   ForEach-Object
Alias           foreach                                             ForEach-Object

When used in the context of an equation, it's the modulus operator:

> 11 % 5

1

and as the modulus operator, % can also be used in an assignment operator (%=):

> $this = 11
> $this %= 5
> $this

1

A post PowerShell - Special Characters And Tokens provides description of multiple symbols including %

% (percentage)

1. Shortcut to foreach.
Task: Print all items in a collection.
Solution.
... | % { Write-Host $_ }

2. Remainder of division, same as Mod in VB.
Example:
5 % 2

% can replace Get-ChildItem | ForEach-Object { write-host $_.Name } which will not work without either the % or the ForEach-Object.