What is a unikernel?

I am new to unikernels and the following links didn't help me much to understand them:

• https://searchitoperations.techtarget.com/definition/unikernel
• https://en.wikipedia.org/wiki/Unikernel

1. When is it better to use a unikernel?
2. How is a unikernel smaller than a microkernel in terms of code size?

In one line, (application + unikernel) called a workload running on hypervisor(cloud) is equivalent to a standalone application running on bare metal in embedded world.

Unikernel in cloud is better when app(workload) dont use most of the OS & device driver services.

Unikernel is compiled with only the used features of kernel NOT with necessary features as in microkernel, hence size is small.