The surprise percentage is basically the difference between the earnings per share (EPS) that the analysts following the stock had predicted/expected versus the 'real' EPS.
For example, if the analysts following a company expect $1.00 per share, and the company reports $1.05, then the surprise percentage is 5%. Very simple, it's just a measure to show wheather there's a big variance between what the street "expects" from a company versus what it reports. Generally if a company gives the analysts guidance from time to time, the expectations tend to be a little closer to the actual results, reducing the surprise percentage.