Double Entry
The ideas of keeping accounting records for an entity and of measuring money are easy to understand. The third idea, double entry, is more complicated. You need to understand this idea clearly because the whole of accounting is based on it.
The main two end products of an accounting system are the income statement and the balance sheet. The income statement reports the entity's financial performance during an accounting period. The balance sheet reports its financial status at a specific point in time. The income statement is by far the more important of the two, but the idea of double entry is easier to understand if you start with the balance sheet.