The column view of the expanded Data panel gives you an overview of the values in a certain column. Here, you can also perform some data preparation and cleansing which can help getting visualizations the way you want them.
The column view will look different for different types of columns.
This example shows a string column. The column name (1) can be changed by clicking on the Rename column button, , and typing the new name. You can click at the link at the top of the column view to go to the data table view (2), which shows an overview of all columns in the data table. Depending on the data type and categorization of the column you can see different statistics (3).
You always have the possibility to change the categorization (4) and data type (5) of the column (but for calculated columns you edit the expression to change the data type).
You may also have the opportunity to change things like the sort order (6) or formatting of the column, or, you may have the option to handle empty values in the column. You can also have access to actions that are specific for the data type of the column, such as the Split column action that can be used to split string columns (7).
To the right in the column view you will see the contents of the column (8), either all values or unique (distinct) values only.