Subordinating conjunctions are used to join a dependent clause - that is a clause that cannot stand alone - to one that can, an independent clause.

EXAMPLE:
a) The whole project will fail unless we put in more money.
b) The tiger must be captured before it killed more villagers.
c) We still feel a bit hungry though we have eaten a lot.

In (a), unless we put in more money is a dependent clause; it cannot stand alone. It depends on the independent clause: The whole project will fail. Here, the subordinating conjunction unless is used to join the clauses.
In (b), the dependent clause is before it killed more villagers.
In (c), though we have eaten a lot is the dependent clause.