Agile methods are those that use an adaptive lifecycle instead of a predictive one.
In predictive lifecycles, the final product of the project is clear at the beginning, and can be planned and designed upfront. In this case, the project goal is to follow the plan and create the product as designed.
Sometimes it is impossible to define the product with certainty at the beginning, because of extreme changes in the market, or the customer. In this case, adaptive lifecycles can be used. In these lifecycles, the development is iterative: each time focused on a subset of the final product. When the subset is complete, feedback will be received and it would be used to identify, plan, design, and execute the next subset.