Certain assets are depreciated for tax purposes. Depreciation is a way to get a tax deduction by spreading the cost of an asset over a period of time. As a result, depreciation reduces the asset's cost basis (adjusted cost basis).
When you eventually sell that asset, the gain on the sale will be higher (than simply sales price minus purchase price) since it's basis is now lower, because of the depreciation. How the gain is treated actually depends on the type of assset.
For people selling residential rental properties. Part of the gain will be taxed as a capital gain will qualify for the lower rate on long-term gains. However ... the part of the gain that is related to depreciation will be taxed at a maximum 25% rate. The technical term for gain related to depreciation on residential property is called unrecaptured section 1250 gain.
ON the plus side... when a rental property is sold, any passive activity losses that were not deductible in previous years become deductible in full. This can help offset the tax bite of this depreciation recapture tax.
As a "heads-up," some people try not to claim depreciation as a strategy to avoid the recapture tax. But this doesn't work, because the tax law requires depreciation recapture to be calculated on depreciation that was "allowed or allowable" (Internal Revenue Code section 1250(b)(3)).
So, good tax planning says you really should claim depreciation on your property to get a current tax deduction, since you're going to have to pay tax on the gain due to the depreciation, regardless of whether you actually took the depreciation deduction or not.
Hope this helps