Open an enemy up in fighting games means bypass the opponent's defense and hurt them.
There is however several ways to open an opponent's defense independent of what games or mechanics the same provides. It can be either you attack them and they fail to block, or you can force a specific situation in the game that favors your counters
The first think you need to know is that defense in most cases means blocking but also space control and screen dominance is also viewed as defense.
In a basic perspective, players who tend to block low can get opened up with a overhead and players who block standing can get opened by lows.
In an adept perspective, you should realize that someone guarding isn't actually attacking and is mostly focused on defense, players who block everything are prone to get open by throws, which by pass any guard since they're expecting mixup options such as overheads or lows, except if the game is designed with anti-throw mechanic like UMK3 or Trilogy.
And on a way more advance perspective, open someone up doesn't always mean you force yourself into the opponent, the opposite also works like counters for example.
It can be from either baiting risky moves, or lure your opponent into a specific momentum of the game where you have complete control and counter them there where they cannot guard everything.
Without going too much in detail watch a few seconds of this video:
The Jin player knows he has some strong mixups if some of his tools are hit or blocked, but everytime he tried to use them he left himself open for more than 14 frames which were enough for Asellas to sneak an eletric wind god fist with steal the entire momentum by taking the game into the wall where his character gains way more powerful options while opponent chances of escaping gets limited by the wall.
Attacking doesn't always mean its going to hit, the most advanced players in competitive tournament are able to see through obvious choices and play around it
