Data source Blocks are used to interact with one of your connected data sources. For example if you have connected Hubspot, you will see Hubspot listed in the Data Blend editor under "My datasources". When you click Hubspot, you will see a list of available Blocks:
Each block performs one specific action, e.g. reading data, updating data or triggering an action (e.g. "Follow someone" for the Twitter data source).