Q:

what is apt.conf in simple and understandable slang? 

I have gone though man apt.conf.  I feel the information there is quite overwhelming and difficult to understand for beginners moving to Ubuntu/Debinan Platforms. 
Could some one explain in simple english what is the use of this file and how it can benefit an apt user. 
Further this file seems be not setup on my machine - so I think it's not required for normal usage, but I am trying to understand advanced usage.  so want to dig in to it. 
ll /etc/apt/apt.conf
ls: cannot access '/etc/apt/apt.conf': No such file or directory

Edit:
I feel trying to understand the apt conf part on a whole makes sense as every thing should be some or the other way related. 
On going to /etc/apt and doing an ls -l
/etc/apt$ ll
total 72
drwxr-xr-x   7 root root  4096 Aug 28 23:46 ./
drwxr-xr-x 144 root root 12288 Sep  1 15:53 ../
drwxr-xr-x   2 root root  4096 Sep  1 15:53 apt.conf.d/
drwxr-xr-x   2 root root  4096 Apr  9 15:51 auth.conf.d/
drwxr-xr-x   2 root root  4096 Apr  9 15:51 preferences.d/
-rw-r--r--   1 root root  3412 Aug 31 20:35 sources.list
drwxr-xr-x   2 root root  4096 Aug 31 20:34 sources.list.d/
-rw-r--r--   1 root root  3410 Aug 31 20:35 sources.list.save
-rw-r--r--   1 root root 14771 Aug 28 23:46 trusted.gpg
-rw-r--r--   1 root root  8497 Aug 27 01:28 trusted.gpg~
drwxr-xr-x   2 root root  4096 Aug 28 23:46 trusted.gpg.d/

Evey thing here is understandable after bit of digging.  Except for apt.conf.d/ (and  auth.conf.d/, which seems to be empty and I am not bothered about this in this Q). 
Now on entering - apt.conf.d/ and doing ll you see bunch of more files -
/etc/apt/apt.conf.d$ ll
total 96
drwxr-xr-x 2 root root 4096 Sep  1 15:53 ./
drwxr-xr-x 7 root root 4096 Aug 28 23:46 ../
-rw-r--r-- 1 root root   49 Aug 18 01:22 00aptitude
-rw-r--r-- 1 root root   40 Aug 18 01:21 00trustcdrom
-rw-r--r-- 1 root root  630 Apr  9 15:51 01autoremove
-r--r--r-- 1 root root 1349 Sep  1 15:53 01autoremove-kernels
-rw-r--r-- 1 root root   92 Apr  9 15:51 01-vendor-ubuntu
-rw-r--r-- 1 root root  168 Aug 31 20:32 10periodic
-rw-r--r-- 1 root root  108 Apr  2 16:55 15update-stamp
-rw-r--r-- 1 root root   85 Apr  2 16:55 20archive
-rw-r--r-- 1 root root  168 Aug 31 20:32 20auto-upgrades
-rw-r--r-- 1 root root  243 Dec 16  2009 20dbus
-rw-r--r-- 1 root root 1040 Mar 12 18:50 20packagekit
-rw-r--r-- 1 root root  114 Jun  5 18:43 20snapd.conf
-rw-r--r-- 1 root root 2592 Jan 18  2020 50appstream
-rw-r--r-- 1 root root 2164 Feb  9  2019 50apt-file.conf
-rw-r--r-- 1 root root  625 Oct  7  2019 50command-not-found
-rw-r--r-- 1 root root 5459 Apr 14 04:07 50unattended-upgrades
-rw-r--r-- 1 root root  435 Jan 18  2020 60icons
-rw-r--r-- 1 root root  251 Jan 18  2020 60icons-hidpi
-rw-r--r-- 1 root root  182 Aug  3  2019 70debconf
-rw-r--r-- 1 root root   32 Aug 28 12:34 99synaptic
-rw-r--r-- 1 root root  305 Apr  2 16:55 99update-notifier

So through this Q, I am looking for answers regarding the significance of apt.conf and the set of files in apt.conf.d/ - what kind of configurations does these files enable ( and of course man apt.conf is not quite understandable at least for beginners)
Second Edit following user535733 response -
How are the files in the apt.conf.d named.  Do they follow some naming convention that simplifies understanding what config aspects go where. 

A:

[I]n simple english what is the use of [apt.conf] and how it can benefit an apt user. 

The configuration files (apt.conf and apt.conf.d/*) allow users to customize how various apt functions work.  Common customizations are for local proxy servers and for the Unattended Upgrades feature. 

Users benefit from fine-grained control if they need it.  Most users don't need it.  Most users never need to touch these files.  The default settings work for most users. 

Some referenced files may not exist on your system.  That's okay.  Different config files are placed/removed in the apt.conf.d/ directory as-needed by various pacakges (or by you).  That's why the apt.conf.d/ directory exists - so applications can make changes without editing a single master config file. 

Files in the apt.conf.d/ directory are loaded in alphanumerical order.  That's why the file names in that directory include numbers.  The order is important: If two files try to change the same setting, the last file loaded wins. ( If you discover two files trying to change the same setting differently, please file a bug report against those two packages so the developers can evaluate and fix the conflict.)