It seems like a term that gets thrown around by people who don’t define what they mean first. 
every FG has more or less the same fundamentals, adding some new stuff (parrying, guard cancel, deflect, chicken guard, issen, beginner-middle-upper mode, power-speed-ex mode) etc, it is just that they are different in execution. Jumping in one game would be the wrong choice in another in the same situation.
I’d say it is easier to carry fundamentals from one Capcom game to another Capcom game or from one SNK game to another SNK game, than from Capcom to SNK games or vice versa. Or from Melty Blood to Blazblue.
It does not mean that being good in fundamentals of SF2 will make you good at SSIV or Guilty Gear.
Most everything that cna be considered fighting game fundamentals can be be summed up in one word: footsies.
Footsies covers:
-when to attack
-what attack to use (poking/counter-poking)
-where to attack from (spacing)
-when to defend
Basically, the whole concept is a series of “what” and “when” questions. Being good at fundamentals means being able to answer these questions correctly in just about every situation.
Fundamentals are easy to understand. You use them all them time.
For example, knowing your and your opponent’s characters’ moves. This is basic knowledge. You have to know their ranges, properties, and of course how to perform them.
Performing them ties in with another fundamental skill - execution. If you can’t perform your character’s moves, you can’t really play them effectively.
Reaction is another fundamental skill. Not just raw reaction speed, but also identifying all possible responses, and choosing the best one.
So consider how all of these things fit together in a simple scenario:
- Your opponent throws a fireball
- You can see from the speed he throws it that it’s the fast version (character knowledge)
- From your position, you recognize that you can either jump forward, straight up, block it, match it with your own fireball or perform an invincible move through it (reaction + character knowledge)
- You decide to match it with your own fireball which builds some meter and leaves both players at a relatively neutral position (reaction + execution + character knowledge)
The harder parts are applying them on more subtle levels, and/or in situations where there is not much room for error. You can break it down roughly into 3 levels.
Consider the scenario where Ryu is standing half a screen away from another Ryu. Most newer players will recognize that it’s impossible for either of their sweeps to hit each other. However, as you move the characters closer to the maximum range of his sweep, more and more players will begin to doubt themselves. Top players however can identify things like this within a very small margin of error. You can apply stuff like this to basic timing as well (eg. “if an opponent jumps over a fireball at this point, can Ryu DP them in time?”).
That’s basically level 1 of mastering fundamentals. Level 2 is applying it in a match. It’s much easier said than done because the opponent is not going to stand around and wait for you to count pixels/frames.
Level 3 is probably the hardest and most complex - Using your fundamentals to help achieve a particular objective
Here’s a simplified example:
You decide that your best strategy of a certain matchup is to score a knockdown. So you navigate your way into the space where you can tempt your opponent to try to poke you with one of their moves, hoping that they will misjudge the distance or timing leaving you with an opportunity to sweep them and score a knockdown. Seems simple enough, but can you react in time, and how confident are you that you maneuver safely in and around that space? And what if they decide to jump instead? That’s where the other fundamentals come into play. You need to know that are able to react to it, and what you can do to punish the jump. If they just keep moving back? That’s good too, because now they’re closer to the corner which is usually beneficial for you. If they stand their ground and challenge you? Well then it just depends on who has the best understanding of the ranges/options involved, and reactions + exection (all fundamental skills).
This is just scratching the surface, but hopefully you have a better idea of things you can work on to improve your game at a fundamental level. Do some research about footsies and zoning, and see how some of these ideas apply to the game you’re playing and the character you are using.
Thanks for all the replies, guys! You’ve given me a lot to think about and I think that after reading your responses I’m already on my way to improvement.