Staffs,
The bar associated with an FPL is on the approach to the toe of a point, the bar prevents the FPL being returned as a vehicle literally stops the bar raising and thus locks the lever.
A clearance bar, as Stationmaster has said, is a similar device in its operation but is not connected with a FPL - at Hooton North for example there were bars withing the double junction to prevent the signalman accidentally changing the points with vehicles on them.
hth