Hard sci-fi is when writers take time to understand current science and understanding how things would work, and then apply it to the future. Arthur C. Clarke is the default example of hard sci-fi.

Basically, "hard" sci-fi uses real world science to figure out how something would work in a future setting. And hard sci-fi really tries to figure out if something is practical outside of a set piece. "Soft" sci-fi is more about social problems of the real world and beyond, like Star Trek. But there isn't an exact formal definition for where hard starts and soft begins, and vice versa.

And I think 95% of scifi fans would agree that neither is better or worse, it just fits the story as its needed. Personally I love hard scifi as a concept, but my favorite scifi stories are all soft, like Star Trek.

https://en.wikipedia.org/wiki/Hard_science_fiction