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)
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.