Prior depreciation equivalent is the amount of depreciation that was "allowed or allowable" from the first date your business started using that asset. It is the total for all years.
AMT stands for Alternative Minimum Tax. The amount of depreciation for AMT purposes may be different than the prior depreciation if you are subject to AMT. The most simple explanation is if you took and Special Depreciation allowance or Accelerated Depreciation on the asset in prior years, that special (or excess) amount would be deducted for AMT purposes. If you did not do this, then AMT Depreciation would be the same as regular depreciation.
Also, please see this link regarding AMT Alternative Minimum Tax Common Questions .