In this election circus one candidate struggles stay on message while the other struggles to stand up. Much of the swirl around Clinton focuses on her use of a private email server, with the intention of keeping her emails private, even after she left office. That probably would have worked if it weren’t for the Benghazi debacle and the investigation that uncovered the server.
This calls into question her judgment, her character, and her willingness to be open about what she is doing while serving the country. However, while not good judgment, according to the FBI not illegal and for her nothing to worry about.