This is the 14977th most frequent Russian word.
подписавшие
"Подписавшие" means "those who signed" in English.
Here, "подписавшие" identifies the group of individuals who completed the action of signing the petition, using it as a descriptor.
Лица, подписавшие петицию, требуют изменения закона.
The persons who signed the petition are demanding a change in the law.
"Подписавшие" is used as a descriptor for individuals who performed the act of legally signing the documents.
Подписавшие документы подтвердили свое согласие.
The ones who signed the documents confirmed their agreement.
In this sentence, "подписавшие" functions as a past participle used adjectivally to specify entities involved in the action of signing the contract.