The ld50, which is the dose of a substance that will result in death of 50% of the people who take it, is not really a medical term. It is a term from the science of toxicology.
In medical use, it is a way judging the relative safety of a medication. Generally, a physician would want to prescribe a medication with a therapeutic dose that was as much lower than its ld50 as possible..
Most substances, even water, can kill people if taken at a high enough dose. The ld50 is simply a way of looking at the relative safety, or danger, of a substance.