Powershell argument

Is there any specific meaning for "%" and $_ in the following powershell ?  Thx

$AllMailboxes = Get-Mailbox | ?{$_.ExchangeUserAccountControl -eq 'AccountDisabled'}
$AllMailboxes|%{$_|New-MailboxExportRequest -FilePath \\<server FQDN>\<shared folder name>\$($_.Alias).pst}