According to Avina, the Relay Monument is a popular topic of discussion for Citadel academics and scholars. There are two theories behind its meaning. The Monument may have been a gesture of Prothean vanity, proclaiming their galaxy-wide mastery of mass effect field technology. Or, the Monument may be a symbol of unity, an acknowledgement that the mass relays would eventually lead other races to the Citadel and unite them.
If spoken to nearby, Commander Shepard's squad all have their own opinions on the Monument.
Conduit, is on Ilos. Together they create a 'back door' onto the Citadel, which hidden Prothean scientists who survived their race's annihilation used to circumvent the signal the Reapers use to control the Citadel, in scant hopes that it would lead to the cycle of reaping being broken for the next races.
Saren Arterius intended to use the Conduit to bypass the Citadel's defenses and hand over control of the station to Sovereign. Shepard used the Conduit to follow Saren in the Mako, and emerged through the Relay Monument on the Presidium, crushing several geth in the landing. As the Conduit was already closing when Shepard drove through, it is likely the Relay Monument is inactive once again.
Two years after Saren's invasion, the Monument is still on the Presidium and is now referred to as the Conduit by the Presidium Groundskeeper.