Grain of a Dimension can be defined as the lowest level of summarization possible for the dimension. Both dimension and fact will have their own grain associated. To understand the grain we need to understand about the columns in the dimension table. Column in the table will have a relation with the other columns in terms of child parent relationship. So when you search for the grain of a table then it should be the lowest summarization that is available for the table.
Grain of the table is important while retrieving the data from the facts or dimensions