A participle is a form of a verb used as either an adjective or a part of certain tenses. There are two types of participles: the present participle and the past participle. The present participle ends in -ing and the past participle usually ends in -ed, -en, etc.
In the sentence you provided, there are two participles:
sent: This is a past participle that modifies the noun refrigerator. It describes the action that the refrigerator had done in the past.
signed: This is also a past participle that modifies the noun order. It describes the state of the order that she had.