Every computer application, development UI toolkit or software architecture has a grain – while anything may be possible, some things are easier than others, subtly shifting you towards these easy paths. This can be used deliberatiley to nudge users to use prefered options.
Understanding the grain of the platforms and toolkits that you or those in your team use can help you to understand what design options will be easier or harder, and where extra care may be needed during development. For example low-level notification-based event mechanisms mean that it is easy to implement a non-model dialag, but extra work is requred to create a modal dialog.
The term 'grain' here is refering the the grain in wood and the way it is easier to 'go with the grain' when working it. It is not the same as 'granularity', whch refers to the scale one is considering.
Used in glossary entries: modal dialog, software architecture, UI toolkit