taken from Euclid's Elements, Book 1
The famous Parallel Postulate, or Postulate 5:
That, if a straight line falling on two straight lines makes the interior angles on the same side less than two right angles, the two straight lines, if produced indefinitely, meet on that side on which are the angles less than the two right angles
What could this possibly mean? Let's take it apart.
If a straight line falling on two straight lines- so we have three lines
makes the interior angles on the same side less than two right angles- so the interior angles formed sum to less than 180 degrees
then the postulate says that those lines will intersect.
This seems obvious (that's why it's a postulate): Because if the sum of the angles is less than 180, then let's say that angle equals x. Then if we have another angle that's 180−x, the three angles sum to 180, and they form a triangle!
If the lines form a triangle, they aren't parallel.
If the sum of the angles were greater than 180, the sum of the angles on the other side would be less than 180, so they would intersect.
In fact, the parallel postulate is equivalent to the fact that the sum of the angles in a triangle is 180 degrees.
Another equivalent statement is that
"Given a line l and a point P not on that line, exactly one line goes through P parallel to l"
(I know this was very wordy for a geometry diagram. The parallel postulate was so controversial for this reason)