There are two distinctly different kinds of planning.
In linear planning we start with our assumptions usually in the form of predictions and expectations about the future, but they are assumptions nevertheless. Then we form a plan based on these assumptions, not taking into account uncertainties that will certainly occur as we move forward. And then we try to implement the plan meaning trying to follow it based on our assumptions.
And because change is a constant in the universe things change. Unpredictable things happen unplanned. After we admit plan failure we either adjust or abort the plan. We hear people talking about following a plan it is usually code for linear planning.
In agile planning we start with our new unknowns. This is easy because every planning context is abundant with more unknowns than knowns. Out of these we form new questions and through these questions we gain new learning. Learning comes about through decision-making, research or some kind of experimentation.
Out of our new learning we then engage in new action which leads to another cycle of new unknowns, questions, learning and action. Agile planning is a cycle of iterations where questions and learning we take actions that are distinctively realistic because they're not based on assumptions but rather in the ever churning nature of reality.