The Mean is the average in everyday English. It's calculated by adding up all the values in the sample and then dividing by the number of values.
The Median is the number for which half of the values in the sample are higher than it, and half are lower. The median of a list of numbers can be found by arranging all the observations from lowest value to highest value and picking the middle one. If there are an even number of observations, the mean of the two middle values is often used.
If Trend Lines Are Erratic, they merely represent the actual prices: the fewer properties of a particular type are listed in a certain location and month, the more scope there is for several extreme prices in amongst them raising or lowering the average noticeably. The most recent month's data may be incomplete: a further cause for volatility.