I see an 'S'-column in the transactions screen. What's normally in that column?

The column shows transaction state, Account balancing (or Reconciliation) is based on transaction lifecycle which is defined by transaction states. Every transaction has three states (statuses): uncleared, cleared and reconciled. The first status basically means that the transaction has been entered manually and you don't have any confirmation of it from the bank. Once the bank reports it at the web site or in the statement, the transaction can be moved to the next state: Cleared. Once you reconcile all cleared transactions with the bank statement, all of them should be moved to the Reconciled state. The reconciled balance should be the same as the last statement balance.