Upside

What Does Upside Mean?
The potential dollar or percentage amount by which the market or a stock could rise. This is basically an educated guess on how high a stock could go in the near future. 

Investopedia explains Upside
The upside can be derived through either technical analysis or fundamental analysis techniques. The estimate is used to get an idea of the attractiveness of an investment. The greater the potential dollar or percentage rise, the bigger the upside for the investment.

For example, if a company has risen 100% in a short period of time based on a new product line, some people may feel that the company has limited upside in the near future as it has already had a strong run.