What does the "@" symbol do in Powershell?

PowerShell will actually treat any comma-separated list as an array:

"server1","server2"

So the @ is optional in those cases. However, for associative arrays, the @ is required:

@{"Key"="Value";"Key2"="Value2"}

Officially, @ is the "array operator." You can read more about it in the documentation that installed along with PowerShell, or in a book like "Windows PowerShell: TFM," which I co-authored.

In PowerShell V2, @ is also the Splat operator.

PS> # First use it to create a hashtable of parameters:
PS> $params = @{path = "c:\temp"; Recurse= $true}
PS> # Then use it to SPLAT the parameters - which is to say to expand a hash table 
PS> # into a set of command line parameters.
PS> dir @params
PS> # That was the equivalent of:
PS> dir -Path c:\temp -Recurse:$true