Board Thread:LotR Mod Technical Support/@comment-26295802-20150721204831/@comment-151.51.58.197-20150815121448

This error tell's you that minecraft need more ram than what is currently allocated.

You have to allocate more ram to minecraft to solve the problem.

Here is a tutorial: http://www.minecraftforum.net/forums/support/unmodified-minecraft-client/tutorials-and-faqs/1871637-tutorial-allocate-more-memory-for-minecraft