Does anybody know what an end of ljne resistor does in a fire alarm system?

If you use the Search tool, you will find this:

https://www.askmehelpdesk.com/electrical-lighting/purpose-behind-resistors-274767.html

It is used to induce a trickle current in the alarm loop. That way, the alarm panel can detect the wire being loose, broken, or melted during a fire and trip the alarm.

The Resistor used as an end of line resistor is needed to show an intact circuit, without it, if you had a short, open or fault across the pair, it would not be seen without the resistor. It MUST be within a couple of hundred ohms(more or less depending on Panel) or it will show as a fault.