Helles is a Bavarian style beer, a type of lager, and the most popular beer in Munich. It has a light golden color, and is more aromatic and less bitter that most other beers. The most famous brand abroad is Löwenbräu (although Munich locals often consider it as being at the lower end of the spectrum of the seven traditional Munich Helles brands).
The word Helles is short for helles Bier (meaning pale beer)-- in order to distingush to Dunkles Bier (dark beer), which is another type of beer typical for the region, being darker in colour and sweeter than Helles.
For completeness, these two types of barley-based beer should be contrasted to Weissbier, a wheat beer also very popular in Bavaria, and also available in light and dark variants (helles Weissbier and dunkles Weissbier).
In Asia Tsing Tao has a flavour very similar to Helles.