The Sages define prayer as a service of the heart, and they consider it a substitute for the bringing of offerings after the Temple’s destruction.

“To love the Lord your God, and to serve Him with all your heart and with all your soul” (Deuteronomy 11:13). Which is the service of God that is performed in the heart? You must say that this is prayer.

“I am sleeping, but my heart is awake” (Song of Songs 5:2). “I am sleeping,” from bringing the offerings; “but my heart is awake,” for the reciting of Shema and prayer. “I am sleeping,” from the Temple; “but my heart is awake,” for synagogues and study halls. (Ta'anit 2a; Shir HaShirim Rabba 5)