She was using her own private email address and email server, located in her home in upstate new york, while she was Secretary of State.
The reason this is a problem is that it means she was not using the government system which would back up her emails and save them for accountability purposes.
She should have been using a government email address, plain and simple, but she set up a server so she could delete email whenever she wanted and keep her interactions secret.
Tying this in with the fact that her brother got a VERY lucrative mining contract with Haiti while she was Secretary of State and the fact that The Clinton Foundation recieved donations from foreign countries (foreign countries with bad human rights records) while she was Secretary of State, though she’d promised not too take foreign donations during her time in that position, and suddenly people are starting to realize how corrupt she is.
Not that anyone who’s been paying attention for the last 2 decades (or grew up in Arkansas like me) doubted the corruption of the Clintons for a second.