Estimating and planning are critical, yet are difficult and error prone. We cannot excuse ourselves from these activities just because they are hard. Estimates given early in a project are far less accurate than those given later. This progressive refinement is shown in the cone of uncertainty.
The purpose of planning is to find an optimal answer to the overall product development question of what to build. The answer incorporates features, resources, and schedule. Answering this question is supported by a planning process that reduces risk, reduces uncertainty, supports reliable decision making, establishes trust, and conveys information.
A good plan is one that is sufficiently reliable that it can be used as the basis for making decisions about the product and the project. Agile planning is focused more on the planning than on the creation of a plan, encourages change, results in plans that are easily changed, and is spread throughout the project.