This property is applicable for only Custom Grid and Grid entry forms.
To do this, select the DataGrid, go to Properties Tab, and in Design section, first make the Readonly = False and then RowSelectType = Cell.
Then, in Misc. Section, go to Columns property, click Collections, Select the respective column and set the property Readonly=False for making the cell editable. If you don't want the cell to be editable, let Readonly = True
For more info, please see the form Sal_Outstanding_Bills in SampleApp.ert. Check the grid columns.
(Sample ERT File is available In Kushal Installation Folder i.e. C:\Kushal\Samples or By clicking on Sample Menu)