Suppose you have three letters a,b,c. I'd like you to make some arrangement of these letters. For example:
abaaaaacbaabc
Now we make strings out of this by taking the n-th to the 2n-th term and lay these strings out (we don't take strings that go from n to k<2n, just leave those last characters ignored):
ab,baa,aaaa,aaaac,aaacba,...