Analyze g Statistics (F6)

The statistics command brings up a window with several statistical values of all selected data sets. If you draw a selection rectangle before selecting this command, only the data points that fall within the rectangle will be counted*.

The contents of the window are also placed on the clipboard, for easy pasting into other programs.

The meaning of some of the statistical parameters:

Mean: the average, or arithmetic mean, which means the sum of all Y values divided by the number of data points.

Median (or Q2, or middle value): the value below which 50% of the data points fall.
Q1:
the value below which 25% of the data points fall.
Q3: the value below which 75% of the data points fall.

Mode: the value which occurs the most. A maximum of 20 modes is calculated, if several different values occur an equal number of times. If all values occur only once, there is no mode, in which case "no mode present" is displayed.
The program calculates the modes with values that are absolutely equal. If you want to include values that are approximately equal, it is better to calculate a histogram of Y values, where you can specify bands of  Y values. 
If statistics is calulated for more data sets at once, only the first mode of each data set will be shown (if multiple modes are present within one data set).

Interquartile mean: the mean of all data points that fall between Q1 and Q3. Can be useful to ignore extreme values (spikes for example).

Mean of absolutes: the average of the absolute values of all Y values.

RMS: Root Mean Square value. To calculate RMS: square all the values, take the average of the squares and then take the square root of the average. Is used mostly for trigonometric functions (sinuses etcetera). For example, your AC mains voltage is indicated in RMS.

Harmonic mean: 1 / (the sum of the reciprocals of all Y values, divided by the number of data points).

Geometric mean: The n-th root of the product of all Y values, where n is the number of data points. Used for example to calculate an average annual return of interest.

Variance:  , where N is the number of data points. When calculating the variance of a sample, N-1 is used instead of N.

Standard deviation:
the square root of the variance. It is the most common way to measure spread.

Semi-interquartile range: Another way to measure spread: (Q3-Q1)/2. Since half the values in a distribution lie between Q3 and Q1, the semi-interquartile range is 1/2 the distance needed to cover 1/2 the number of data points.

Some good web sites on statistics can be found here, here and here.