Greyscale is a disease, which is left untreated can infect a person's entire body and drive them to madness. Jorah contracted the disease on season five and Daenerys finally found out after she had decided to trust him again. 

Daenerys banished Jorah once more, but this time with good intent. She ordered him to go find the cure for his greyscale and return to her in Westeros to be by her side once again.