I saw it mentioned on this question, but no one explained what it is.
ROAM stands for "Real-time Optimally Adapting Meshes." It is a level of detail algorithm for rendering large terrains. It's somewhat complicated so I'll link to some more in depth explanations:
Here is the paper: https://graphics.llnl.gov/ROAM/roam.pdf
The following is a slightly less academic explanation: http://www.yaldex.com/game-programming/0131020099_ch14lev1sec4.html