Active High and Active Low

It has two inputs:
Trigger (555 pin 2) makes the output high.
Trigger is 'active low', it functions when < 1/3 Vs.

This means that this pin works when the voltage is less than 1/3 the Supply Voltage so this pin is connected to the 0V Rail to discharge the chip power - see diagram on link:

http://www.kpsec.freeuk.com/555timer.htm#bistable

Pin 2 is connected to the 0 rail.

For instance, if you have an IC with an active LOW reset pin, you would need to keep it HIGH during use of it, and make it LOW when you wanted to reset.

Active High means that the output of the chip (or a condition in the chip) changes when the input is made high.

Active Low means that the output of the chip (or a condition in the chip) changes when the input is made low.

source: http://forum.allaboutcircuits.com/showthread.php?t=10619