It’s the percentage of time a core in active, when not idle or shut down. That’s confounded by Intel Hyper-threading, and frequencies of Apple silicon cores.
% CPU
The CPU view in Activity Monitor is the starting point for tuning the performance of software. Here are its virtues, and a few vices to beware of when using it.
One of the most common reasons for opening Activity Monitor is to check the % CPU of processes that might be running out of control. How accurate are those figures, though, and should you believe them?
The Eclectic Light Company 