Passive means that the subject is receiving the action. For example, in your sentence "saccus portatur a servo", the saccus is being carried. In an active sentence, the direct object receives the action ("servus saccum portat"). Latin passive is equivalent to English "be ____ed" (e.g. "be killed" as opposed to "kill").
flebile nescio quid queritur lyra, flebile lingua murmurat exanimis, respondent flebile ripae