To build effective tables with Grid.js, you need to understand how to define columns and provide data. In Grid.js, the columns array determines the structure and labels of your table, while the data array supplies the actual content for each row. Each entry in the columns array can be a simple string for the column's label or an object that specifies more options, such as a label and a field for mapping. The data array is typically an array of arrays, where each inner array represents a row and its values correspond to the columns in order. This mapping ensures that each data value appears under the correct column label, creating a clear and organized table.