This place description refers to the site where Bhagwān Ṛṣabhadev (Ādināth), the 1st Tīrthankar, attained Keval‑jñān (omniscience).

• Pūrimatālā / Purimtal – an ancient suburb associated with Ayodhyā (later known as Prayāg/Prayāgraj).
• Under the Akṣhayavaṭ (eternal banyan) tree – here Bhagwān Ādināth performed deep tapas and attained Keval‑jñān.
• This is therefore the Keval‑jñān kalyāṇak-bhūmi of Ādināth Bhagwān in the present time‑cycle, and the place where he first preached, formed the four‑fold saṅgh (sādhu, sādhvī, śrāvak, śrāvikā), and laid the foundations of Jain Dharma. ( en.wikipedia.org)

You can read a simple history of this tīrth here: “Shri Purimtal Tirth” and a brief story form here: “History of Shri Purimtal Tirth” .