“He just released it over the weekend.”
You're telling your friend about a new song by an artist you both know, which was put up on YouTube on Friday. He doesn't know about it. You're explaining it, and you say this.
He just released it over the weekend.
One of the uses of "just" is to express that something happened very recently. It can mean a few minutes before:
I'm sorry, he just walked out to go get lunch.
Or immediately before:
What did he just say?
Or it can mean a few years:
Mankind has just started to study ways to understand and control DNA.
But in any case, "just" means a short time before.
To "release a song" means to send the song out to the public, so that people can hear it and buy it. You can "release" songs, movies, books, and software. Another phrase with a similar meaning to "release ___" is "introduce ___". There's not a really strong difference between the two, but if you say that something was "released", it sounds like people wanted it a little more and were waiting for it. To "introduce" something sounds like more work! Also, individual people can "release" a product, but "introducing" a product feels like it takes a larger organization of people. Here are some examples of both:
Sony has hinted that they may be introducing a new tablet computer at next week's conference.
Kanye is releasing a new album in the fall.