This slide illustrates how a pair of iterators can be used to describe a collection. Imagine, for example, we have an array of values of type Card. We might describe an array of 52 cards as extending from the address cards to the address cards+52. What is iteresting about the latter address is that it is past the end of the collection, it is the next value in sequence after the last one. The past the end value is not part of the collection, but is just a marker for the end.
The STL generalizes this idea, using pairs of iterator elements to describe a range of values.