Declining balance method of depreciation is a technique of accelerated depreciation in which the amount of depreciation that is charged to an asset declines over time. In other words, more depreciation is charged during the beginning of the life time and less is charged during the end.
Why more depreciation is charged in beginning years? The reason is that assets are usually more productive when they are new and their productivity declines gradually. Thus, in the early years of their life time, assets generate more revenue as compared to the revenue generated in later years of their life. According to the matching principle of accounting, we should depreciate more of the asset's cost in early years to match the depreciation expense with the revenue earned from the use of the asset.