c++ - What Do Double Subscripts ([i][j]) Mean? -

i know vague question , possibly horribly worded, beginner in c++ , keep seeing (btw copied code off of someone's post):

if ( contents[i].size() >= 5 ) {         ( unsigned int j=3; j<4; ++j ) {             double value = contents[i][j];             contents[i][j] = 0.; } } 

what "[i][j]" part mean in contents[i][j] = 0.;? familiar using 1 [i], i've never used two... can tell me how using 1 [i] different using [i][j] , means use [i][j]?

sorry vague question, couldn't find answer this. thanks

content stores 2d array: array of array (probably std::vector< std::vector<double> >, thank @owacoder comment).

contents[i] accesses i's array of contents, std::vector<double>, , contents[i][j] acesses j's elements of i's array stored in contents.