I assume you mean in the reports tab in an activity, correct? Daily will be your absolute conversion rate for each day in the time series graph. Per the documentation, “Running Average” reflects the cumulative conversions (from the start of the reporting window to the date represented on the graph) divided by the cumulative visitors.
So let's say I have the following conversions and visitors by date. Note the difference between the the "absolute conversion rate" and "running average conversion rate" columns.