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: 
For a rough estimation, you can use Load Average value shown on htop right upper corner. Value more than number of the cores is a sign that the system is overloaded, which is apparently true in your case as 42 is greater than 12.

For more info about system load and LA particularly a good read is [here](http://www.brendangregg.com/blog/2017-08-08/linux-load-averages.html)
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.