A workbook is your container to build and transform your data. You can add DataSources, create calculations and build visualisations.
DataSources and calculations are built in an excel-like framework called a DataGrid and visualisations (graphs etc.) are built in the visualisation section of a workbook. The data (Datasources and calculations) that you select and manipulate in a DataGrid will “drive” what you see in your visualisation, so if you remove a column from your DataGrid, that series will disappear from your chart.
In the image below, “A” is the visualisation section where you create the visualisations you need to add to your dashboards. “B” is the excel-like functionality that allows you to manage your data.