Understand the rule/rules behind this code for cellular automata?

Questions
1

Hi all,
I need to understand the rule/rules behind this code;

Table[ArrayPlot[
CellularAutomaton[{i, {2, 1}, {1, 1}}, {{{1}}, 0}, {{{30}}}], 
ImageSize -> Tiny], {i, 2, 20, 4}]

if anyone can explain the parts of this code, which one is the rule number or what i stands for?? excuse my ignorance. I am new to coding and trying to read patterns from different codes to match plan geometries of architecture plans.

thanks
Maryam