C++ References let you give variables alternative names. For example, if you write
  int & jones = flossie;
then "jones" becomes an alternative name for flossie in the current scope. If you assign something new to jones, you're not going to make the variable into an alternative name for something else - you're actually going to change the value that's being held under (both) names. You can see a worked example in [source code here].

Why do you want to give variables alternative names in this way?

One reason is that you're wanting to loop through every member of an array - perhaps an array of objects too, where the variable names to describe a memory location are complex. Rather than repeat the complexity time after time in your code, a reference is a lighweight alternative to a pointer to simplify it. For example, you might write:
  for (int i=1; i<length; i++) {
    Shape * ¤t = candidates[i];
    sofar = current->heavier(sofar);
and you can then keep making further references to current rather than the more complex candidates[i] for the rest of the loop too.

There's a further example that compares and contrasts references and pointers [here], and one that uses both reference and pointers to array members [here]
(written 2010-07-02)

 C234 - C and C based languages - Further C++ Object Oriented features
  [801] Simple polymorphism example - C++ - (2006-07-14)
  [802] undefined reference to typeinfo - C++ error message - (2006-07-15)
  [831] Comparison of Object Oriented Philosophy - Python, Java, C++, Perl - (2006-08-13)
  [1159] It can take more that one plus one to get two. - (2007-04-22)
  [1819] Calling base class constructors - (2008-10-03)
  [2004] Variable Scope in C++ - (2009-01-22)
  [2005] Variables and pointers and references - C and C++ - (2009-01-23)
  [2576] What does const mean? C and C++ - (2010-01-15)
  [2673] Multiple Inheritance in C++ - a complete example - (2010-03-12)
  [2717] The Multiple Inheritance Conundrum, interfaces and mixins - (2010-04-11)
  [3057] Lots of things to do with and within a C++ class - (2010-11-16)
  [3069] Strings, Garbage Collection and Variable Scope in C++ - (2010-11-25)
  [3124] C++ - putting the language elements together into a program - (2011-01-08)
  [3238] Bradshaw, Ben and Bill. And some C and C++ pointers and references too. - (2011-04-09)
  [3430] Sigils - the characters on the start of variable names in Perl, Ruby and Fortran - (2011-09-10)
  [3509] Operator Overloading, Exceptions, Pointers, References and Templates in C++ - new examples from our courses - (2011-11-06)
  [3982] Using a vector within an object - C++ - (2013-01-19)
  [4366] Changing what operators do on objects - a comparison across different programming languages - (2014-12-26)
  [4377] Designing a base class and subclasses, and their extension, in C++ - (2015-01-01)
  [4559] When do I use the this keyword in C++? - (2015-10-29)

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Further C++ material - view new or old
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What are C++ references? Why use them?
C course - final course example puts it all together
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Objects and Inheritance in C++ - an easy start
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