A subordinating conjunction, also called a subordinator, joins a dependent / subordinate clause to a main verb. They are used to show any relationship between them and they turn the clause into something that is dependant on the rest of the sentence for its meaning.

For example:-

Because it was snowing ... makes no sense on its own.
We went skiing, because it was snowing. Aha!

Be warned, there are a lot of them.

Subordinating conjunctions include: after, although, because, before, if, in case, so that, therefore, unless, when, while ...

For example:

Before you came here, you thought you understood English grammar.
We left the party when the police arrived.

!Note - The subordinate clause sometimes comes at the beginning of a sentence.