It means that a septic system (for the sewer) can't be installed because of soil conditions.
Property has to absorb water (Perk) in order to have a septic tank installed. The tank requires a "drain field" in order to empty its contents.
To perk a lot, you dig a hole, fill it with water and count the time it takes to drain. If it drains too long then it doesn't perk.
A septic system is basically a series of pipes underground where your waste water drains into. If the soil is too hard to let it drain, the system will not work.
However, depending on where you live, the Health Department, may allow other systems like a sand mount system such as raised engineered system.