Estimation
In plain terms
Predicting the effort or time a task will require.
Going deeper
Estimation forecasts how much work an item involves to support planning and prioritization, despite inherent uncertainty. It informs commitments.
Under the hood
Techniques include story points, t-shirt sizing, and planning poker; estimates are forecasts, not promises.
Examples
Story points, planning poker, t-shirt sizing, forecasting, sprint planning.
Think of it like...
Like judging how long a trip will take before setting out.