"Backfill" in Medved Trader means filling up historical daily or intraday data for a particular symbol. This is done either explicitly by user request or could be executed by default (for example, upon  opening a chart if the "Auto Backfill" option is checked in the Settings / Charts / General.

 

The source for the backfill data can be either explicitly specified or be implicit. The implicit - default - source for backfill data should be set in the Dashboard by clicking on the Backfill Source or the Hist BF source button.