What is the meaning of the saying in the Hadiths: "Prayer is a congregation" (الصلاة جامعة), and does it mean the call to prayer or to gather then speak?

"Salat Jami'ah" means "gather for prayer," or that congregational prayer is now being held. The Prophet, peace and blessings be upon him, used it to gather people for prayer or for important matters, as in the story of Al-Jassasah. After the legislation of the Adhan for the five obligatory prayers, this phrase remained for non-obligatory prayers, such as the eclipse prayer.