The Active SQL tab of the history viewer displays active sql statements (requests) over the course of the selected timeframe.
The graph and grid at the top of the screen displays the number of active statements at a point in time. Clicking on the graph will highlight that sampled minute in the grid.
Clicking on a statement in the grid will display all active requests occuring at that point in time in the next grid in the middle of the screen:
If more than 1 request was occurring it allows you to select an individual request and then view its T-SQL and execution plan in the lower part of the screen: