I finally know what the above two words are and how to use them on a Linux machine.

SIGTERM - Send an application a signal to terminate itself the way it normally would, i.e. let it pack its bags like it does. A request to empty its place on RAM.

SIGKILL - Just kill the application. No bullshit. Shoot it with the sniper. No confusions.

How to use them:

You need to find the process id for of the app you want to kill -15 or kill -9, i.e. terminate or kill, respectively.

to find the pid (you can just type part of the app's name here):

pgrep appname

or, if you know the exact console name of the app:
pidof exact_appname

Then, kill that app

kill -9 pid

Or, politely ask it to terminate

kill -15 pid