When a company asset gets used over its life, it generally tends to lose value, the reduction in this value is called depreciation. Rather than recording all of the expense at once for something you use over an extended period of time, accumulating the depreciation is a way of recognising its value over the period of time you use it for.
The accumulated value of all the depreciation on your assets. Remember this amount could be across a number of years for multiple different assets.
What's not included
Impairment of an intangible asset.
The writing down of any assets when sold (this should be recognised in Disposal of Assets)