The value entered is not compatible with the properties of the column. For example, if you enter NULL into a column that does not allow null values, or if you enter text into a column with a data type of Boolean, the changes will not be committed to the database. The error message will continue to appear until you either press ESC to revert to the original value or you enter a value into the cell that is appropriate for the column.
To see the all of the properties of a column, in Object Explorer right-click the table with the column properties you want to view, and then click Design (Modify in SP1 or earlier). In the table grid of Table Designer click the column. Its properties appear in the Column Properties tab below the table grid.