In statistics, replication is repetition of an experiment or observation in the same or similar conditions. Replication is important because it adds information about the reliability of the conclusions or estimates to be drawn from the data. The statistical methods that assess that reliability rely on replication.
For example, if you select a person from the population of a city and measure his/her body height and weight, this leaves almost no room for statistical methods. But replication of this observation through random sampling of, say, 100 more individuals from the population of the same city, changes the situation radically: now it is possible to draw some conclusions about weight, height and their interrelation in the city population at large.