How do I pass an array or list as a parameter to a PowerShell function?

Try this:

function CheckCert([string[]]$ComputerNames)
{
    $deadline = (Get-Date).AddDays($global:threshold) # Set deadline date
    foreach ($computer in $ComputerNames)
    {
        Invoke-Command -ComputerName $Computer { Dir Cert:\LocalMachine\My } |
        foreach {
            If ($_.NotAfter -le $deadline)
            {
                $_ | Select Issuer, Subject, NotAfter, @{N="Expires In (Days)";E={($_.NotAfter - (Get-Date)).Days}}
            }
        }
    }
}

Working with PS 4.0 or later, it´s possible to define as CheckCert([array]$ComputerNames) as well.