The A string is the most important string on the violin. All the strings are important, but the A string is the note that you tune to. So, when an orchestra is tuning, the oboe gives an A to the concert master.
That's the A. Here's an example of a piece on the A string.
The beginning of 'Meditation' by Massenet is mostly on the A string, as you heard, the beautiful string on the violin.