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

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

Code:

median = J(1,3,.)

mean in the code

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?

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