What it does is change the consonant sound into an -OR sound.
HUH? Okay, H'm ... let me show then.

The consonant you see above are BAR, SAR, HAR, and AR. (respectively)

But when we have the swiggley line, the SUK GAR, above it we change the -AR into an -OR sound.

So....

BAR changes its sound into BOR

SAR changes its sound into SOR

HAR changes its sound into HOR

AR changes its sound into OR

Do I have to remember all that, Picklefish?

Absolutely not! All you have to remember is how to use the SUK GAR. Here is a trick.
First you look at the consonant and then say it's name. For instance, you see the letter GAR you say, "GAR". And then you notice that swiggley-thingy, since you already know what it does to the consonant, just change its sound into an -OR ending.

This "backward" like -C is call SUK DAR,  which literary means "hair of the consonant DAR".
You'll usually find this symbol above a consonant; followed by the YOR consonant.
Whenever you see the SUK DAR arranged like this, it makes a long "I" sound or pronounced '-ai'.
You might want to remember this sound by saying the word: PIE. Yeah, as in APPLE PIE, CHERRY PIE, or whatever  your favorite PIE is.
H'm, let's see how we apply this pronoucation to some of the word with the SUK DAR   mark.

Let's read it together.
NOR + sat ET(vowel) + SAR + SAR(subsript) + SUK DAR(above) +YOR
NES -- SAIY
The word "NESSAIY" means to desire,understanding, or to have in common.

You might have noticed that I've sounded out the YOR + sat AH(vowel) differently.
YOR + sat AH should be YAH, isn't it?
I've have shown you the rules of pronoucing some of the vowels differently with the consonant that has an -OR sound at the end. For example, the consonant GOR -KOR -NGOR -JOR - CHOR -NYOR -DOR -TO -TDOR -TOR --NOR - BPOR-- POR -- MOR -YOR -- ROR -WOR -.
the sat AH becomes sat EER
the sat AY becomes sat ET

Let's read this word together.
We see the vowel sat-A(long -a song as in DAY), now let's move to the first consonant SAR.
SAR + NOR(subscipt) + sat A + HAR + TREY SAB
SNA
The word SNA in Khmer means " love charm".

Now let's read an similar word.
SAR + NOR (subscript) + sat A + HAR + sat AH
SNAHA
The word SNAHA means (poeticly) to LOVE or LOVER.

The T'MANG GAN-DOU appears in the following consonants.

What the T'MANG GAN-DOU does is change the -OR consonant sound into an -AR.

(Hint: it is used the same way as the SUK GAR but this time it is the -OR that changes into -AR.) HUH? (scratching your head, and picking your nose)

Hey , stop that. Okay let me explain.

There you have (above) are the consonants NGOR, NYOR, BAR, MOR, YOR, ROR, and WOR, respectfully.

So, ... 

NGOR changes it sound into NGAR

NYOR changes it sound into NYAR

BAR changes it sound into BPAR   (see explaination below)

MOR changes it sound into MAR

YOR changes it sound into YAR

WOR changes it sound into WAR

Okay, what is up with BAR??
There is an exception to ever rule.
Since BAR does not have an -OR sound and it's special because changes its sound to BPAR.

Okay, let's read this one.
BPOR + NAR + robat(above)

BPOR  (nar is cancelled by the "robat") meaning "color".

Here is another word that uses the "robat" mark.

Let's read it together.

BPOR + (TDAR is cancelled by the "robat") + MOR + sat AH(vowel) + NOR

remember sat AH becomes sat EER when it follow an -OR consonant , in this case MOR

BPOR MEN which means  "news or currect events"

This mark is called "ASH DAH". (don't ask me why)
This mark looks a lot like the number 8 in khmer script. 
This mark only appears in these two word below.

GAR and DAR

Whenever you see the GAR consonant with the number 8 above it; it's a word within itself.
GAR like this means "also, so, therefore or then".
DAR means "that or which" whenever you see it like that.

Yeah, you just read the letter and know how to use the number 8 mark.

This mark is called "BAN TDUK". This mark usually appears above a letter.
Whenever you see this mark ; you must make a short -U sound, ask in the english word "ugly".
The short -U sound comes before the letter beneath it.

Let's read it's name.

BAR + NOR + TDAR(subscript) + GAR + Ban Tduk (above)

BAN TDUK

See?