A unikernel is simply an application that has been boiled down to a small, secure, light-weight virtual machine. The resulting virtual machine image does not contain an operating system like Linux or Windows. There are no users and no shell to login. Since it is one application it prevents other applications from running by design.
Unikernels are widely considered to be the next generation of cloud infrastructure for their speed and security.
Be sure to check out NanoVMs on StartEngine!