Okay, I have enough searching and now I'm just totally confused.
I am not a professional musician and I don't know music terms and I can not read notesheets so bear with me. One day doing web-surfing about some music I came across a comment about song that I really like. Comment was like "Oh, I like this song, because it is so upbeat!".
"Okay" — I thought, — "I don't know what does it mean, something with beats, perhaps".
After that day I've stumbled over this kind of comments again, and again, and again, so at some point I start thinking that "upbeat music" is some kind of genre. Genre that I like, obviously.
Recently I saw comment about Megalovania in the same manner: "It is so upbeat". Curiousity finally got me and I start searching term 'upbeat' only to increase my confusion.
So, google says that upbeat is "an unaccented beat preceding an accented beat". It's just one beat, right? How the whole song can be upbeat? Further research give me this: "The downbeat is the first beat of the bar, i.e. number 1. The upbeat is the last beat in the previous bar which immediately precedes, and hence anticipates, the downbeat.". Upbeat at the beginning also called "anacrusis".
Okay, how this can be related to the whole song? Further research tells me that upbeat is when conductor do upward stroke, and downbeat is when conductor do downward stroke. Well, I don't understand anything in conductor's gestures, so it means nothing to me.
And the final piece was: "In a measure considering: 1 and 2 and 3 and 4, “1, 2, 3, 4” is the downbeat, while the word “and” is the upbeat". I'm done. I need help.
So, I asked my collegues. They aren't professional musicians either. One of them suggested: "May be upbeat is just means cheerful and happy?". Well, this may make some sense, but I personally can't call Megalovania a happy or cheerful song, especially considering context of this song in a game - it's battle with Sans and it is one of the most dramatic or even tragic moments in a whole game. How can someone call this "happy?". Other upbeat songs I like also came from boss battles.
Is "upbeat" music means just "happy, cheerful, optimistic" music?