What does "matrix median = J(1,3,.)" mean?

[Phuc Nguyen]

I am trying to draw a graph based on imported coefficients and confident intervals, thanks to the help, I found this documents.

Regarding the part "Plotting results from matrices" . I am wondering what does matrix

median = J(1,3,.)

mean in the code

 sysuse auto, clear (1978 automobile data)
 matrix median = J(1,3,.)
matrix colnames median = mpg trunk turn   .
matrix CI = J(2,3,.)
matrix colnames CI = mpg trunk turn  
matrix rownames CI = ll95 ul95
 local i 0

Apart from that, I am confused why matrix median is J(1,3.) and matrix CI is J(2,3,.), whether they are overlapped?

[Leonardo Guizzetti]

You would do well to familiarize yourself with looking up the documentation for commands and functions that you don't understand by now.

help matrix
help J()

The function J() creates a matrix with defined dimension and insert a default value. Median and CI seem obvious in their meaning: one stores medians, the other stores the confidence limits (upper and lower limits, so 2 values per variable). Both matrices here are devices with a system missing value by default.