This means exactly what it says. Fifty percent will survive this diagnosis and fity percent won't. Survival depends on stage of cancer at time of diagnosis.
It means that in their experience, people with cancer at a similar stage and progression as yours and with similar health conditions live and survive the disease (usually for a set number of years) about half of the time. It is often reported as 5 or 10 year survival.
It means that half of the people who are in your situation will die, while half will survive. Basically, it means that your cancer is extremely serious. Your doctors are telling you that they simply cannot tell whether you are more likely to survive or pass away from this disease.