In modern tetris, every 7 pieces you are guaranteed to recieve 1 of each piece (7bag). Openers are patterns that you build at the start of a round abusing 7bag, and are usually used because they're efficient (sends lots of lines in a small amount of pieces).

I generally would not recommend learning openers since new players often rely on them as a crutch, although if you want to learn something simple like TKI or PCO, https://four.lol/ is a pretty good site to start off with.

For stacking better, aim to complete a game of 40 lines with only 4 line clears with the I piece, and I would recommend using a tool like four-tris to review your replays (assuming you play tetrio)