What is a "timer"?

The word has been thrown around quite a bit on here and on other sites in discussions about the NYC Subway? What exactly are they and what purpose do they serve. I have heard there are some areas of the system that have a lot of timers including the 8th Ave express tracks. Why would there be so many in some locations.

From a Rule book issued in 1952 by NYC Board of Transportation,pg 53 "Time Control of Speed"
"Rule 65(a) A time element is a device which enforces a passage
of a predetermined period of time before a piece of equipment can function.
65(b) A Lever Time Release is a device used in connection with a Lever of an Interlocking Plant,to delay for a predetermined period of
time the release of a Lever.
65(c)A Time Signal or Time Controlled Signal is a Fixed Signal having a time element in it's control.
65(e) A Grade Time Signal is a signal to enforce a predetermined speed on a Descending Grade or at other locations where it's required
to Limit the speed of Trains.
65(f)Grade Time Control of Signals is Intended to cause an approaching train to run thru the Grade Time Territory at a predetermined speed,
which if exceeded,will cause the train to be stopped." (stopped by trip arm)
That's the basics.

You can thank Robert Ray (among other things and people) for all the timers...

There were plenty of timers before the Union Square wreck. What that accident created was *MORE* timers to protect against trains operating over curving turnouts at unsafe speeds.

Hence "among other things and people".