What is meant by "fuel flow: on " in checklists?
In the real aircraft that checklist item would mean bringing the thrust lever of the engine to be started up out of cutoff and to the idle position. This would be accomplished by physically moving the thrust lever forward slightly out of the cutoff detent.

However, in the computer flight sim for which the linked checklist provides guidance, the cutoff position of the thrust levers is not simulated. Instead, thrust lever positions below idle command reverse thrust. For this reason, the checklist calls for using the computer keystroke ctrl-shift-F4 to command fuel flow on. This keystroke combination commands mixture full rich in simulated piston aircraft.

In the real Lear 45, fuel flow and scheduling during start is controlled by the Digital Electronic Engine Control (DEEC). For this reason, the start checklist calls for setting the appropriate thrust lever to idle prior to commanding engine spool up in preparation for the DEEC to automatically start actual fuel flow once the appropriate compressor speed is achieved.

It is worth noting that after initially setting the thrust lever to idle—or hitting ctrl-shift-F4 on your computer—the fuel flow indication should read zero. The check for appropriate positive fuel flow comes later in the start sequence.

Compared to setting the thrust lever to idle in the Lear 45, the analogous control command for starting most piston engines would be mixture to full rich and for most turboprop aircraft would be fuel condition lever to low idle or similar.