What is Statistical Significance?
A result is statistically significant when it would be unlikely to arise by chance alone if there were really no difference, judged against a threshold chosen in advance.
Statistical Significance explained
The usual measure is a p-value, compared with a threshold such as 0.05. A p-value below it means that, if there were no real effect, a result this extreme would occur less than 5 percent of the time.
Significance is not the same as importance. With enough data a trivial difference becomes significant, and a large difference in a tiny sample may not. Look at the size of the effect and its confidence interval too.
Example
A new landing page converts 4.2 percent against 4.0 percent; whether that gap is significant depends on how many visitors saw each page.