The open interest in 18 June 2021 expiry $2.50 put options...

The open interest in 18 June 2021 expiry $2.50 put options stands at 12200 contracts as at close of trading today. Each contract represents 99 shares in the underlying RRL FPO. Therefore, 1207800 shares in total or around $3.1 Million in value.

We don't know if the open interest is for options sold - obligating the writer to take the stock at $2.50 if exercised. Or bought - conferring upon the buyer the right but not any obligation to "put" or sell the shares at $2.50 should they chose to exercise their right.

If the stock closes tomorrow above $2.50 the options will expire worthless and the writer will pocket the premium they received upon writing the contracts. Any buyers of the of the options will lose the premium paid.

Call options work in the opposite way with the seller obligated to deliver the stock at the exercise price if exercised, and the buyer conferred the right, but no obligation to buy the underlying security at the exercise price if they chose to do so.

ETO's are highly leveraged products which can blow up your trading account very quickly if used recklessly.
Your broker will require you to acknowledge that you have read the ASX brochure on options before they will open an account for you.