What does it mean for a core to run at different loads in different moments? How does a 10% load differ from a 90% load? How is this number calculated, essentially?
It should be pretty obvious: time spent executing a task / total time.
So over a given interval, 10% load means 10% of that time was spent executing tasks, and 90% was idle.
As far as I know, the "load Average" is the average number of process requiring cpu execution time over 1/5/15mintes.
For example, say you have 1 CPU with 4 cores, any load superior to 4 means you don't have enough computing power to have all process running as fast as they wish.