turbo c coordinates to pixel conversion and random number gereration

I am making a simple game on Turbo C using graphic mode.I want to generate random numbers in my way. I found the technique to generate random number as:

*i=random(10000)%40;
*j=random(10000)%40;

& there is created a rectangle as:

*rectangle(0,0,400,400);

The aim is to randomize the above numbers i&j over the rectangular area but I don't understand how this is done and I can't understand when is pixel technique used in c graphic and when coordinates?

For example: If I want to write my name in coordinate point(50,2) then the co-ordinate technique seems to be used.

I tried to find out but I couldn't figure out about text mode and graphic mode yet.