The A_ part is simply and easy way to identify a built-in variable that ahk makes. These are reserved variables that you can use in any script to make your life as a script writer easier.
You don't have to breakdown and understand each and every letter/symbol.
AutoHotkey is written to be high-level, meaning that it is English readable.
Anything enclosed in % are variables. Don't try to break them down farther than that, just look for where that value is assigned. If the variable starts with that A_ part, then know that ahk is assigning a value to it automatically.
You will find that most commands are very easy to understand, even if you are not familiar with that particular one yet.
The pattern you are looking for is something that you have to develop yourself as you become familiar with a language.
You should start by writing pseudo-code. Outline in plain English (or language of your choosing) what it is you want your script to do, in a fairly general term.
Next, take each section of your pseudo-code and add a bit of detail.
Keep refining the level of detail until you are fairly certain that you could write each section on its own.
Crack open the chm (compiled help manual - included in the ahk installation) and select the Index tab.
start typing in the name of what you think a command should be.
example:
I want to copy a file from one location to the next.
I type in file and get all commands starting with the word file. the 8th item in the list is FileCopy. Select that and read over the syntax of the command and look at the examples.
That is my method for writing scripts and programs, and I have developed it over about four years of scripting, building web sites and learning different programming languages (or attempting to

)
I do this with every script that I write. Maybe not so much the pseudo-code, as I can think out most of a script before sitting at the computer, but I have the help file opened on my second monitor at least 80% of the time while I write something.
I have also found, through my profession as an electrician, that it is really easy to over-think something when trying to learn it, especially if it seems overwhelming at first.
Small steps with strong understanding will get you far, just make sure that you keep stepping, and don't stall out when you are only one or two steps off the ground.