hi,
I will give a quick explanation.
Resistors R1 and R2 divide the 12V, so that there is a 3Volt DC level at the junction of the resistors. This 3V reference is connected to a comparator input [-Vsignal input].
When the input voltage at +Vsignal pin of the comparator, from the 'alarm' [ which could be almost any type of sensor, ie: thermistor, photo diode] is LESS than the Vref on the -Input the OUTPUT of the comparator will be 0V.
This means the relay coil will not be powered [ energised] so the realy contacts will be open. As the contacts are open the Horn gets no voltage and will not sound.
When the 'alarm' input voltage becomes greater than the 3 V ref, the output of the comparator will switch to a high voltage,, say about 10V, so the relay coil will be energised, the contacts will close and the horn will sound.
So the triangle is a COMPARATOR, it compares the Vref and Alarm voltage and controls the relay