What's an International Grandmaster?

I know what a International Master is and I know what a Grandmaster is, but what's an International Grandmaster?

There's no International Grandmaster

International Grandmaster is just a regular Grandmaster (GM). The name is abbreviated a bit, but it is mostly just implied or lost in transliteration. 

I think it is just a more professional way of saying a GM is higher than IM.

Almost no one calls it that, but it is just a regular GM.

Look it up on fide you’ll see that sometimes 

ohh ok

it's what they used to call grandmaster back when being international was a big thing

GMs were International before it was cool.

After the St. Petersburg tournament 1914 the Czar of Russia conferred the Grandmaster title on the finalists Lasker, Capablanca, Alekhine, Tarrasch, and Marshall.
The Soviet Union kept awarding (national) grandmaster titles. Those were harder to obtain than present day FIDE GM. You had to either win second place in the USSR national championship or twice in three consecutive years end in the top 6. Many present day GM would not even have been NM in the USSR.
FIDE awarded international grandmaster titles IGM according its title regulations:
https://handbook.fide.com/chapter/B01Regulations2017 
Russia no longer awards a national GM title, so the I in IGM could be dropped.

International Grandmaster is a title below Grandmaster but higher than FMs and CMs

No, it isn't.

it just sound higher if we say im,but there is nothing like that

I did not know that

@blueemu is correct. I think you are confused with International Master (IM) @paxrosales IM is an official title lower than GM and higher than FM or CM, but this isn't what the op is asking about here xD

Oh ok. Thanks

thank you for telling me

It  is a grandmaster