This alows you to display a simple, maybe default, empty selection in your combo, maybe to allow for "every option" or "ignore this filter", as I'll show you here:

SELECT po.PurchaseOrderId,p.PrefixId

FROM PurchaseOrders AS po

INNER JOIN Prefix AS p ON po.Prefix = p.prefix

WHERE p.PrefixId = @Prefix

OR @Prefix = -1

This OR @Prefix = -1 allows you to pass -1 and actually ignore the filter to return all rows.