Oversold or Overbought are just a rough area in which technicians or chartist use to describe the bullishness or bearishness of the stock.
There are several indicators that can be used to indicate that the shares are oversold. One of the most common ones is RSI. For example, above 70% RSI is considered overbought (bearish) while below 30% is considered oversold (bullish).
Take these regions with a healthy dose of skepticism. What is oversold could remain that way for a long time and you do need a certain level of market experience to see what is considered oversold/overbought for different stocks.