LU stands for Live Update. It is when the LIVE (playable) servers are updated to new patched code. They seem to happen about every 4-5 weeks. Some are bigger then others.
SoE does not like people to use this term anymore, in favor of Game Update, but LU is more fitting because it is when the LIVE servers are updated, not the TEST servers. This distinction is very important for people who have active characters on LIVE and TEST servers at the same time.