colDt_s and colDt_e are used to dynamically pass From and To date in Time Frame section in Report in Salesforce.

Sample Code:

String reportURL = ‘/{!ReportId}?colDt_s={!DateField}&colDt_e={!DateField}’;
Check the below links for dynamic parameter passing to reports in Salesforce

https://www.infallibletechie.com/2014/02/how-to-change-report-criteria.html

https://www.infallibletechie.com/2014/07/how-to-create-dynamic-filter-fields-and.html

https://www.infallibletechie.com/2014/05/dynamic-filters-for-reports-in.html

Cheers!!!