This refers to the fact that when you sell a business asset, you need to "recoup" the depreciation. The IRS doesn't care if you actually TOOK the depreciation or not. If you could have, even if you didn't, you need to repay it.

If you purchased a truck for work for 25,000 and it's a 5 year item, (depreciate 5,000 a year) and you sell it after 4 years for 10,000, you would have a 5,000 gain, whether you took the depreciation on the truck on your taxes or not.