Reserves represent the amount of money ‘owed’ to the owner (shareholder) of the company.
In a profitable and conservative company, reserves will normally comprise a significant balance of accumulated undistributed profits.
These deposits are current account balances in favour of the depositing institution, effectively cash deposited at the central bank.
Central banks may require institutions to maintain minimum balances with the central bank, in which case balances in excess of the minimum are known as 'excess reserves'.
Of course banks publish accounts and use the term 'reserves' in the accounting sense too. Do not be confused by this.