The document discusses depreciation, which is the decline in the value of fixed assets over time due to use, age, or obsolescence. Depreciation is accounted for for tax and profit purposes by allocating the cost of the asset over its useful life. It ensures that the costs are matched to the periods that benefit from use of the asset. Common methods to calculate depreciation include straight-line and written down value. The document also discusses the need to create provisions and reserves to cover uncertain costs and retain profits for future needs.