In numerical analysis, adaptive mesh refinement is a method of changing the accuracy of a solution in certain regions, during the time the solution is being calculated. When solutions are calculated numerically, they are often constrained to discrete locations which constitute the computational grid, or 'mesh'. For example, when calculating a solution to the shallow water equations, the solution (water height) might only be calculated for points every few feet apart - and one would assume that in between those points the height varies smoothly. The limiting factor to the resolution of the solution is thus the grid spacing: there will be no features of the numerical solution on scales smaller than the grid-spacing. Adaptive mesh refinement (AMR) changes the spacing of grid points, to change how accurately the solution is known in that region. In the shallow water example, the grid might in general be spaced every few feet - but it could be adaptively refined to have grid points every few inches in places where there are large waves.
If the region in which higher resolution is desired remains localized over the course of the computation, then static mesh refinement can be used - in which the grid is more finely spaced in some regions than others, but maintains its shape over time.