trial balance

It is opening inventory because your closing inventory from the previous financial statement forms your opening inventory in the trial balance for the current year and the closing inventory value are always gotten after a stock take verification have carried out to ascertain the value of stock using lower of cost and net realization value.
The value gotten is always used as the closing stock value by debiting the closing stock in the statement of financial position and crediting the profit or loss.
This does not always comes into the trial balance because the closing inventory is always determined a day after the financial year end of the company.
(In my own opinion)

headbayo is not quite right – closing inventory should be determined on the last day of the financial year, not the day after.

The trial balance is prepared immediately after the bookkeeping is finished, but before the year end adjustments (inventory, accruals and prepayments, depreciation, irrecoverable debts).
It is prepared to check that the bookkeeping has been done correctly – that the debits to equal the credits.
For that reason, the balance on the inventory account will be the balance that was left there last year – i.e. the opening inventory. It will be a debit balance.

If you have not already done so then you should watch my lectures on double entry booking and trial balance (and the adjustments for inventory etc,)

Great 🙂