Append Changes to Existing Text
I'm sure you've come across a description or comments (multiple line of text) field in a task or issue list that goes blank when you go to edit it - the previous text entry is added as history/version. The setting for this can be turned on or off by going to the column settings and changing the "Append Changes to Existing Text"
In order to enable this feature, you must enable versioning. If you have it enabled and want to disable it be very cautious as you will lose all the previous history entries.