Euclid's Postulates

If two lines are drawn which intersect a third in such a way that the sum of the inner angles on one side is less than two Right Angles, then the two lines inevitably must intersect each other on that side if extended far enough. This postulate is equivalent to what is known as the Parallel Postulate. (math.harvard.edu)

What does which mean or what’s its role in the context?
Is which referring the fact that two lines are drawn, and the fact intersect a third line? Though it’s awkward, I don’t get any good idea how to parse the structure.

It begins a relative clause modifying 'two lines'. If two lines Unfortunately the relative clause is very long, so the rest of the main clause, 'are drawn', would suddenly appear out of nowhere if you put them in the normal place:

If two lines are drawn, then . . .

So these words are brought forward, or in other words the relative clause is moved beyond it. It makes it easier to read.