Subordinating conjunctions make up the bulk of all conjunctions, and their job is to connect sentences. The difference to conjunctions is that subordinating conjunctions “embed” one sentence into another, making it kind of dependent.
Important examples are that, because and if.
In German, they also SIGNIFICANTLY change the sentence structure of the embedded sentence.
That’s a bit annoying but subordinating conjunctions are INCREDIBLY important. Like… you can NOT make normal sentences without them.
Textbooks and courses tend to bring them late, because they’re stupid, but you should learn the important ones as early as possible, even if you don’t get the structure changes right.