Online Mo Algorithm

Tried to learn online mo algorithms by looking at this code-library/MOs Online.cpp at master · ShahjalalShohag/code-library · GitHub code for codeforces 940F. However there’s this part of the code I don’t really understand specifically

int nw = 0;
for(int i = 0; i < BC; i++) {
for(int j = i; j < BC; j++) {
int id = nw;
I[i * BC + j] = nw++;
for(int p = st[i]; p <= en[j]; p++) t[id].add(a[p]);

I think I saw similar mo algo question that also have something like that, isit something like to compute for “jumping of blocks”?