what are openings?

Well, there's the opening "phase" and there is opening "theory"

Basically the opening phase is the part of the game where you try to organize your pieces and get your king safe. In the starting position no one influences any squares beyond their 3rd rank, the kings are vulnerable, and just in general none of the pieces are working together. So typically (not 100% of the time) strong players spend the first ~10 moves without making any big threats, they focus on bringing their bishops and knights off the back rank quickly and castling.

Opening theory is (usually) a sequence of moves by both players that has been played many times in the past because the moves are high quality. Beginners usually get confused and think "moving both my knights for moves 1 and 2 is my opening" but that's not an opening. An opening is something both players do. If you want to play the Sicilian defense and white plays 1.c4 then you can't anymore because the Sicilian happens after 1.e4 c5.

Interesting :)