This is the = command that adds a new column based on a Python expression you provide. You can reference the cells in the current row based on the column name.
It is also possible to create a column based on the output of a shell command.
VisiData is row based, not cell based, so you cannot create expressions that reference data across rows.
It calls them derived columns and you can use python to combine and process other columns.
https://www.visidata.org/docs/columns/#derived