In continue to the definition by Spiel Brickner, which was rather accurate, I'd like to add a couple of things.
While using the word "blyat" as a link word (regardless to its meaning) , there are no rules for where to place this word in the sentence. You may place it anywhere in the sentence as many times as possible, even after each word if you will, and it will still remain correct russian grammar.
The bets anology demonstrating it, is the hebrew word "keilu", which northern Israeli bitches use similary as a link word placing it randomly in the sentence regardless the meaning of the word itself.
(A little exaggerated example, even though you can find russian people talking this way)
Yesterday blyat I went to this party, blyat, and it was so lame blyat that even after I got drunk blyat I didn't feel like dancing blyat.
This word is a link word in Russian language which recently became popular thanks to Russians dominating on many international online resources. Generally it just emphasizes all that was spoken before. But also keep in mind that here's word "blyad" in Russian too and this word means a slut. Some people even spell "blyad" when they use the meaning of "blyat", so there's this subtle hint about how exactly the emphasis is created.
Typically an npc which you find wondering around mindlessly, they also come with the most atrocious English youβve ever seen. They are often funny to observe and can be very volatile.
This russian word is one of the universe's greatest mysterious, including:
What happend to the unknown wonders of the worlds?
What is the true nature of god?
Why god created animals out of meat if he did not want us to feast of them?
etc. so does blyat comes along. The formal known version of this particular word is translated to simply- "whore" "slut" and other words that explain the oldest proffesion in this world. Yet, russians (and latley, Israelis as one, even without a slavic roots in blood) are using this term as a link-word, as a curse, almost anything you can imagine.
"blyat, I messed up!"
"you damn bitch, blyat!"
"I hate you, blyat, and I need something!"
"...blyat. blyat?
-blyat."