To filter a dashboard, you could only add a filter whose values were obtained from a datasource.
You can now add free input-fields. This lets the user enter any value within an input-field. The value is stored within a variable.
This variable can be used to filter your widgets. It is also available within the "compute formula" of your dashboards. For example, you can use this to tune a column's value according to a parameter entered by the user.
And last but not least, if you trigger a Python script from a dashboard, all the variables (included these new ones from input-fields) are not now passed to your Python script.