I got it now.
r - range, can be:
nothing - work on the current line only
number - work on the line whose number you give
% - the whole file
a - argument, can be:
g - Replace all occurrences in the line. Without this, Vim replaces only the first occurrences in each line.
i - Ignore case for the search pattern.
I - Don't ignore case.
c - Confirm each substitution. You can type y to substitute this match, n to skip this match, a to substitute this and all the remaining matches ("Yes to all"), and q to quit substitution.