It sits exactly halfway between your minimum of 4 and your maximum of 23.
The midrange uses only the two extreme values, ignoring everything in between, which makes it very fast to compute but sensitive to outliers.
The midrange (13.5) is within 5% of the mean (13), suggesting the data is fairly symmetric and free of extreme outliers.
Next stepCompare the midrange with the mean and median above. If they differ noticeably, a single outlier may be distorting the midrange as a summary of your data.