j is for a loop in a loop.

It was very common in text books when showing nested loops

int nWhatTheCount = 0;
for (int i = 0; i < 10; i++) { 
    for (int j = 0; j < i; j++) { 
        for (int k = 0; k < j; k++) { 
            for (int l = 0; l < k; l++) { // and on, and on
                nWhatTheCount++;
            }
        }
    }
}