@Sue M. The code being spoken is an owner who either wasn't managing their property and raising rents in small increments over time, or a property in bad condition and doesn't warrant increases in rents in the current condition, meaning you'll have to update the property then raise rents.
Things to look out for when someone is offering a property and the main selling point is it's currently well below market rents.
1) Usually the current owner either doesn't have the money to do the upgrades, or the owner knows when you factor in vacancy during the rehab, cost of the rehab, and new tenant lease-up timelines, your break-even from this scenario, is too far in the future and not worth it, so they sell the property.
2) Another thing some investors make a mistake on is buying one of these properties advertised under the guise of "well below market rents", but the seller isn't comparing "apples to apples". I looked at a property advertised this way, the owner was using comps for apartment buildings that had pools, in-unit laundry, gyms, rooftop decks, etc.. but trying to sell a rundown 12 unit building, no laundry at all, no pool, gym, or anything, but telling potential buyers rent for 2 bedrooms in the area are comparable.
3) Make sure to take a look at the leases if you're going to try going the route of not renewing leases, then doing a rehab, and lease-up at higher rents. Tenant friendly municipalities are not good for this type of strategy, as getting the tenants out typically do not go as quickly as planned. Doing unit turns happen over time, so make sure to factor your rehabs, and unit turns over time when looking at your future cashflow projections.
Keep in mind that if a property is well-below market rents, it's that way for a reason.
Good luck analyzing your deals!