This is the profiler. What the individual percentages tell you is how much time was spent processing each part of emulation at the last time interval. The number represents a (rounded) fraction of the 100% total processing time. The purpose of this is to monitor performance to see where your bottlenecks are.
The core is primary N64 emulation (which, for the sake of brevity, is all the important stuff that 1964 does that the plugins and other various things don't do). So, for example, if you see core: 48% and video: 45% more time was spent in the core than in the video plugin. (about 3% more time). As another example, if you see audio at 4%, then 4% of that last time interval time was spent processing audio.
Most of the time in most games, the compiler number will be at 0%. When you see this number greater than 0, then 1964 is compiling code.
If you have the Speed Limit turned off, there is no idle, so this number will be 0%. If you have speed limit on, emulation will idle to keep sync if your machine is able to run the game over the speed limit. The Idle% number tells you how what percentage of the time is spent idling.
When your statusbar is updated with new profiler data, a new time interval begins and the monitoring is repeated. At least, that's how it's done now in 0.9.9 which is an improvement
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