The assessed value of real estate is an estimate the tax department has made based upon the data they have on your property. These can be contested shortly after they are assessed. I do find that many times they are not accurate valuations. The good news is that the folks in the tax department will take a look at the information you bring them during the period of time they allow after the assessment. If you don't get there within the allotted time (usually a few months) it is difficult to get them to change it. You can have them to come out and take a look to see if there data is incorrect. They will change their assessment after the allotted time during this situation.
I assume, based on the limited information you have provided, it means the current owner and possibly the new owner will be paying taxes based on an assessed tax value that exceeds fair market value. It happens in a declining market.