Question: I have a server, which has 12 Cores, and usage is 42. What does it mean?

As far as I know 1 i 100% of one core. So 42 is 100% of 42 cores?

[![enter image description here](https://i.stack.imgur.com/U2w9s.png)](https://i.stack.imgur.com/U2w9s.png)

Answer: 
Yes the server is overloaded/busy. Load average is not as straightforward just CPU. Other things are mixed in too e.g.tasks waiting for disk i/o to complete etc. You need to install some monitoring and gather data about the overall performance of your system. You can then use this to figure out what is the cause of your load issue.
probably this link can be useful for you:

<https://www.deonsworld.co.za/2012/12/20/understanding-and-using-htop-monitor-system-resources/>

In short:

The system load is a measure of the amount of computational work that a computer system performs. The load average represents the average system load over a period of time. 1.0 on a single core cpu represents 100% utilization. Note that loads can exceed 1.0 this just means that processes have to wait longer for the cpu. 4.0 on a quad core represents 100% utilization. Anything under a 4.0 load average for a quad-core is ok as the load is distributed over the 4 cores.