Need help to understand this statement

Could someone please help explain it to me what does the following statement means? It will be best if you could use some examples. Thanks

An iterator can become invalid whenever a change is made to the underlying container.

Code:

---

vector<int>::iterator i = v.begin();

v.push_back( 20 );

// i can't be used anymore

i = v.begin();

// now i can be used again

---

An iterator is like a pointer into the container, but if you change the container and it moves things around internally so that the pointer doesn't point to where the item is, the iterator is invalid.

Thanks

The statement is, without context, too general, though. Among the properties of the various containers is a description of when, exactly, iterators can become invalid.