Unikernels aren't too complicated conceptually. They're more or less a kernel stripped down to the bare minimum required by a single application. The complete bundle of the minimal kernel and application together is called a unikernel. The uni- prefix means one as in the kernel only supports one userspace application, instead of something like linux, which supports many. The benefits, as mentioned in the paper and in this thread are that you can run that as a vm, since it contains it's own operating system, unlike a container which is dependent on the host operating system. Also, they boot very quickly.