Tetris is about fitting the falling blocks called Tetriminos into complete lines which then disappear and make space for more blocks to fall.
These are 4 square connected shapes that fall from the top of the screen. You have to rotate and move these falling shapes to make horizontal lines with no gaps. Don’t let the stack get to the top of the screen.
When the stack of tetrominos reaches the top of the screen.
Tetris is one of the best games ever. I played this when I was a kid.