Legs are also referred to as tears, arches and rivulets. They are the droplets of wine that run down the inside of the glass after you swirl, like paint dripping down a wall and it can sometimes be had to see and may take a bit of practice to identify it. It is known by many as an indication of the alcohol level and quality, but this is not actually true. It does, however, tell you about its body, power and sugar/alcohol content.It is good to keep in mind that there are other factors like the type of glass and the cleanliness of the glass that can effect the the way the wine interacts with the glass.

The three biggest components in a glass of wine is water, sugar and alcohol. When the wine coats the glass, the alcohol evaporates faster than the water. Gravity pulls the water down the sides, making for “good legs.”The term “balance” is better than legs because it describes how the alcohol, sugar and water blend together to give you the result you wanted. The major components in the wine need to play nicely with others and the alcohol should be there, but it shouldn’t be the main attraction. 

More alcohol means more legs and you don't always want more alcohol because it may not be called for in that specific wine eg Moscato, because it is sweet and subtle you would taste the alcohol if it was too prominent. So you can see that this is a term often used for heavier reds to add to the fact that it is heavy, dark, deep wine and with a decent alcohol content with the more legs it it gets.Here is a quick video by Majestic Wine in the UK that will give you a brief overview.