Pre-programmed move input. So you'd never miss a combo or need to actually input a move to do one. Despite this however it doesn't make you invincible. Unsafe moves are still unsafe moves, and consistency is only one piece of being a strong player.
Is super annoying though.
No, it doesn't make you invincible. What it does though, is make the other player realize that any punish= max damage. You have to adjust your gameplay and play super safe, on top of that if they have a bind for a DP, the reaction time to DP goes down, and your ability to sneak in jumps diminishes. The effects of macroing go beyond simple comboing, you have to switch to a turtle style to beat it, or find some read the other person does every time (the good macroers won't do this, it doesn't win you games by itself, but it allows you to win when you shouldn't plenty of times)
Bad players with macros are still bad, average players become very strong with macros.