Below are several external resources to teach you about System Latency - it's definition and usefulness. Nvidia's introduction of their "Reflex" system gives a great overview of system latency.

I must give credit where it's due. Many people smarter than I have come before me and forged my path ahead.

Mark Rehjon and his site BlurBusters.com
Alexey Nicolaychuk (aka Unwinder) - Wrote RTSS housed on Guru3d.com

Prominent YouTubers that have covered system latency in depth:

Chris on his channel Battle(non)Sense
Steve Burke & his crew on his channel Gamers Nexus
Linus Sebastian & his crew on his channelLTT

And here's some of their most important videos regarding input latency:

Battle(non)Sense' Original system latency test method.
2 great videos from Gamers Nexus: