On websites such as Reuters an "analyst consensus" is displayed in the form of a buy-hold-sell sliding indicator.
What is this indication based on?
The indication is based on the average Buy-Hold-Sell rating of a group of fundamental analysts. The individual analysts provide a Buy, Hold or Sell recommendation based on where the current price of the stock is compared to the perceived value of the stock by the analyst.
Note that this perceived value is based on many assumptions by the analyst and their biased view of the stock. That is why different fundamental analysts provide different values and different recommendations on the same stock.
So basically if the stock's price is below the analyst's perceived value it will be given a Buy recommendation, if the price is equal with the perceived value it will be given a Hold recommendation and if the price is more than the perceived value it will be given a Sell recommendation.
As the others have said this information IMHO is useless.
To dig a little deeper, a number of analysts within (and without) Reuters are polled for their views on individual stocks and markets on buy-hold-sell. The individual analysts will be a varied bunch of fundamentalists, technical, quant and a mixture of the three plus more arcane methodologies. There may be various levels of rumors that aren't strong enough to be considered insider trading, but all of these will give an analyst an impression of the stock/market.
Generally I think there isn't much value there, except from the point of view if you are a contrarian trader, then this will form a part of the input to your trading methodology.
It is simply an average of what each analyst covering that stock are recommending, and since they usually only recommend Hold or Buy (rarely Sell), the value will float between Hold and Buy. Not very useful IMHO.