Make a formula field with this formula:
IF({Stock} <= {Threshold}, "need to order")
Then ask yourself if you need to do this with an Automation at all. You could just use that^ field above (and adjust it to account for other statuses as well). But if you want to keep your original Status field, then make your Automation trigger “when record matches conditions” and the condition is “Formula contains ‘need to order’”
Add a Update Record step that changes the Status field.