Let’s begin – first off, it’s on the wiki, here.
When hovering over the performance graph, a tooltip is displayed with an Error and Unstable Rate.
Due to the way the DT (Double Time) and HT (Half Time) mods are implemented, the error and unstable rate values will be multiplied by the same factor as the song. To get the true values when playing DT, divide the results by 1.5. Similarly, multiply the results by 1.33 when playing HT.
Error will always display two values which represents how far off the early hits were on average and how far off the late hits were on average. The higher the Overall Difficulty value of the beatmap is, the lower the Error values will have to be to do well when playing the beatmap.
Unstable rate represents the consistency of the timing of the hits, where lower numbers are better (top players often score below 100). Note that the value measures consistency, not accuracy, so consistently in hitting 15ms early is the same as consistently in hitting “on time.” The formula is essentially the standard deviation of the hit errors (in milliseconds) multiplied by 10. Sample code is available as reference, showing how osu-stable calculates unstable rate.
You ever finish playing a map, and notice the graph that shows your HP over the span of a map? (A simple line graph, showing 0-100% of the map).
If you highlight it, it shows you some interesting statistics – one of which is called Unstable Rate. Here is the official page on it:
So – what do I need to know?
In a nutshell – lower is better.
The lowest is currently around 35ms, held by Corin. Here is a reddit post: https://www.reddit.com/r/osugame/comments/8lqcyh/new_osustandard_ur_record_by_corim/
Unstable rate is, in essence, a measure of how close the Unstable Rate bar (at the bottom of the screen) is to the middle, consistently.
For an example of this bar, check out this terrible video of me playing this song – it’s the Hit Error bar at the bottom.
Also, in general, the better a player is – the lower their UR is (to an extent) – higher OD (OD10, for example) will usually be accompanied by a fairly low UR (eg; sub 110).
Jump maps (with little finger control needed, and a consistent/easy to follow rhythm) will also have, most of the time, a lower UR. Tech maps, slider spam, etc will usually have higher UR.
How to convert?
(Unstable Rate shown in graph) ÷ 1.5
150.00ms, at the end of a DT map
divide by 1.5
'Converted' (also called 'cv') UR is 100.00ms
(Unstable Rate shown in graph) ⋅ 1.33
100.00ms at the end of a HT map
multiply by 1.33
'Converted' (also called 'cv) UR is 133.00ms