For example, if you owe $200,000 on your current mortgage(s), and your home's value has decreased in the last few years, as it has for a great percentage of homeowners in this country, to let's say $150,000, then you're "upside-down"... or "underwater").
The real estate market of the last several years has put many homeowners in a position to have an upside-down mortgage.
So, does being upside-down on your property prevent you from being able to sell? In most cases, no. You very likely could sell by using a short sale. Call me if you'd like to lean more about a short sale and how it could work for you.