That post is part of a sequence called "Map and Territory" [1] in which Eliezer explains that your beliefs are a map, which may or may not correspond to reality, the territory which the map is supposed the represent.
In this post, Eliezer is mocking philosophical arguments about the meaning of "truth", to illustrate that truth isn't that complicated: a belief is true if and only if it corresponds to reality. Eliezer has recently written a much clearer version of this viewpoint [2], in which he again uses the idea of walking off a cliff to illustrate the difference between strong beliefs and true beliefs.
[1] http://wiki.lesswrong.com/wiki/Map_and_Territory_(sequence)