"Speculative fiction" is hard to define, mostly because nobody agrees on the meaning. Broadly, there are two useful definitions, but to understand them, we have to take a brief (BRIEF!) look at the history of science fiction.
1. About 100 years ago, people called science fiction a thing.
2. About 80 years ago, sci-fi hit what's considered it's "Golden Age."
3. About 60 years ago, a LOT of people were writing sci-fi. Not all of it was good.