In plain English
Take the high and low of the last four years. 0 means you are at that low, 100 means you are at that high. In between, you are some percent of the way across the range.
People nickname certain fractions Fibonacci (23.6%, 38.2%, 61.8%). Here the oscillator is just that 0–100 position in the rolling four-year range. Low is historically “cheap vs the recent range,” high is “expensive vs the recent range.”