The word Tefillah comes from the word ‘Pilul’, which means to think. “Re’oh panecha lo pilalti”, “I did not think I would see your face” (Breishis 48:11). L’hispalel actually means to cause oneself to think.
When we daven, we implant and internalize certain ideas in our mind. And that’s why Chazal called Tefilah “Avodah Shebalev”. The ‘lev’ in Chazal always refers to the mind, and that’s exactly what Tefillah is, the way we serve Hashem with our mind.