It should mean 'surplus stocks', which raise fixed costs for storage, management etc. and reduces cash flow.

This is also known as 死蔵品 or デッドストック. Overstock is 過剰品. The dividing line between dead stock and overstock[ed items] is sometimes blurry, but basically you can overstock items that are selling well (you just bought more than was selling, or expected it to be a fantastic seller), whereas 停滞品 or デッドストック are items which just aren't moving because nobody is buying them anymore. Think walkmans, legwarmers, and paisley shirts. _Any_ stock is overstock with those items, and that's why they are 停滞している.