When running simulations, for instance a weather forecast, climate model, etc, it is often beneficial to integrate observation data into this computer model to improve the quality of the final result. For instance, many weather forecast systems integrate data obtained by satellite imagery.
OpenDA is a framework for using Data Assimilation in simulations, without having to implement this in the simulation itself. This allows scientist to make use of data assimilation techniques without implementing this from scratch every time. It also allows for more advanced data assimilation techniques to be used, and easy comparison between different techniques.