This was explained in a previous topic:
On the Vibhaṅgasutta – About the 4 Jhanas
When one does insight mediation, one first gets to samadhi, and eventually gets to the Arahanthood, as described in the Upanisa Sutta (SN 12.23):
“..With the comprehension of suffering (i.e., the First Noble Truth via Tilakkhana) faith results; with the growth of faith, lightness of mind (pāmojjaṃ) arises; with increasing lightness of mind, joy (piti) arises; with increasing joy, lightness of the body (passaddhi) arises; with increasing passaddhi, bodily sukha arises; with increasing bodily sukha, samādhi arises; with samādhi, yathābhūtañāṇadassana (knowledge and vision of things as they really are) arises; with the knowledge and vision of things as they really are, one loses attachment to worldly things (nibbidā), followed by losing cravings for sense pleasures (viragā), and liberation (vimutti), and to the destruction of all defilements (khayeñāṇaṃ)”.
Now, jhana may be attained anytime around the step, “with increasing passaddhi, bodily sukha arises“.
– One may proceed all the way to Arahanthood without getting to jhanas.
That is from my earlier post.
This piti is different from mundane joy. It comes from the mind “being released” from kāma assāda.