It shows on overview of your progress during learning - from it you can see for example if you got distracted towards the end of your study session etc. CurrentAvg indicates the average event in the current session. A positive value indicates you are getting more right that wrong, a negative value you are getting more wrong than right. It is recorded as a horizontal line across the graph. History indicates events in the history of learning that wordlist.
History is defined as what has happened to the loaded wordlist before it was last loaded.
Removing this file will throw away any record of the statistics history. The individual events are recorded as dots. MovAvg30 indicates the moving average of the last 30 events in the current session, as above. CurrentAvg indicates the average event in the current session, as above. HistoryAvg indicates the average event in the wordlist history.