"Work on your fundamentals"

"work on your fundamentals" means that there are very general, very common situations in fighting games that you haven't thought about/practiced yet.

Assuming you're talking about MK11, this can fall under many possibilities. But in short, someone who says that likely won due to a basic strategy

As an example, if you opponent just jumps 10 times at you, will you simply uppercut them 10 times and win the round? Good anti-air reactions are a fundamental FG skill.

Another example, if you block jacqui's f314, it's your "turn" based on the frame data. Do you always take your turn, and do you mixup how you do so? (Do you always down+1, or do you sometimes throw/neutral jump/etc?)

"playing" doesn't make your fundamentals better. Only "understanding" does. So as odd as it sounds to say, what you have to do is seek out things you don't understand, then try to understand them from both a technical and psychological perspective.

It can be helpful to think of the most mundane things like "throws" or "walking" and ask yourself these questions:

1) Why am I doing this? What purpose does this serve?

2) Can this make my opponent play differently? How? What would I do in response to those things?

3) Is this move reactable? If I believe it is, can I react to it?