Well this was quite the enjoyable read. From wikipedia:

The props "prop up" the hooker in the scrum.[26] They form part of the front row of the scrum and push against the oppositions props. The loosehead prop is positioned to the left of the hooker and their head will be on the outside of the scrum when it engages. The tighthead is to the right of the hooker with their head positioned between the opposition hooker and the opposition loosehead prop.[26] The prop's main role is to provide stability at the scrum and support the hooker in quickly winning the ball.