The scene with the Architect is a key pivot for the plot of The Matrix Reloaded and the trilogy as a whole.
Up until this point the underlying premise of the movies are that:
- The Matrix is a control mechanism to maintain their stock of human 'batteries'
- At some point in the past, someone spontaneously awoke from the Matrix
- That person started freeing others and formed the city of Zion
- There is a prophesy of 'the one' who will end the Matrix, and who has Matrix superpowers
- Morpheus (and some others) believe Neo is 'the one', and he exhibits these powers
The Architect reveals that 'the one' is just another mechanism of control, and that there have been many cycles. Each cycle ends with the destruction of Zion and 'the one' selecting a number of human's to rebuild it.
The Architect gives Neo a choice:
He can go to 'the source' allowing Zion to be destroyed and rebuilt.
Or he can refuse this path. He is told that the Matrix will eventually crash, killing all connected humans, and Zion will be destroyed anyway.
In each case he is told Trinity will die.
There are two doors. The door to your right leads to the Source and the salvation of Zion. The door to your left leads back to the Matrix, to her... and to the end of your species. As you adequately put, the problem is choice. But we already know what you are going to do, don't we? Already I can see the chain reaction: the chemical precursors that signal the onset of an emotion, designed specifically to overwhelm logic and reason. An emotion that is already blinding you to the simple and obvious truth: she is going to die and there is nothing you can do to stop it.
So, you ask what is the Architect, and what role does he play in the movie.
Simply put the scene with the Architect dramatically changes the course of the trilogy. Neo is told that he has a role, but that role is just another mechanism of control of the humans. He isn't some grand savior of humanity as many believe. He's been told that he is destined to be the sixth person to start the building of Zion, perpetuating the Matrix and the imprisonment of humanity, or the person who finally dooms humanity to extinction.
Neo of course chooses to disbelieve the Architect or trust that he can overcome the consequences of attempting to break the cycle. He chooses the left door, to re-enter the Matrix and attempt to rescue Trinity.
What has changed? Well he is now even less certain that he was before about why he has his powers, whether he can trust The Oracle any more, and whether he has chosen to doom humanity.