When you have decided on your cut-over or conversion date from the old system to ShowHouse, you'll need to effectively take a snapshot of your business accounts at close of business on the old system. This is called a trial balance and you may need the assistance of your accountant with this one-time setup process.
Your conversion balances will define the state of play in terms of the various accounting balances on the new start date, e.g. the money in your bank account, the rent and charges due from tenants, the client monies held for payment out to landlords.
If you are a new agency starting from scratch your conversion balances will be zero so you won't need to worry about this.
Read more on entering your conversion balances.