What is it? And how to use it?
is it considered cheating to use the opening explorer?

I don't think so. Opening books, paper or otherwise, are permitted. Plus, the opening explorer doesn't always give the best moves, just the moves others have played--something I've discovered in postgame analysis.

I, like a fool, don't use anything. Just play as if playing on a board. Have always played like that.

I, like a fool, don't use anything. Just play as if playing on a board. Have always played like that.
same!
It's allowed in daily chess: https://support.chess.com/article/317-what-counts-as-cheating-on-chess-com.

I agree it's not a good idea to use the explorer beyond a few moves. I've made the mistake of following a line too far, only to find myself in an inferior position or a position I don't even understand.

A chess engine takes a specific position, analyzes it, and suggests the best moves. In other words, a computer program that plays chess (usually at grandmaster strength).
Why not just use OUR skills instead of some explorer or whatever.
Feel free to do so, but using databases during the game is a good way to learn (of course you are not supposed to just pick the best scoring move, but instead use the database to understand the available plans). Also, afaik, daily chess has been designed as an online platform for correspondence chess, where books have always been allowed - for obvious reasons.
I use it sometimes in daily games, and i dont know if it is considered cheating.
so is it?