  We initiate the study of Boolean function analysis on high-dimensional
expanders. We give a random-walk based definition of high-dimensional
expansion, which coincides with the earlier definition in terms of two-sided
link expanders. Using this definition, we describe an analog of the Fourier
expansion and the Fourier levels of the Boolean hypercube for simplicial
complexes. Our analog is a decomposition into approximate eigenspaces of random
walks associated with the simplicial complexes. Our random-walk definition and
the decomposition have the additional advantage that they extend to the more
general setting of posets, encompassing both high-dimensional expanders and the
Grassmann poset, which appears in recent work on the unique games conjecture.
  We then use this decomposition to extend the Friedgut-Kalai-Naor theorem to
high-dimensional expanders. Our results demonstrate that a constant-degree
high-dimensional expander can sometimes serve as a sparse model for the Boolean
slice or hypercube, and quite possibly additional results from Boolean function
analysis can be carried over to this sparse model. Therefore, this model can be
viewed as a derandomization of the Boolean slice, containing only
$|X(k-1)|=O(n)$ points in contrast to $\binom{n}{k}$ points in the $(k)$-slice
(which consists of all $n$-bit strings with exactly $k$ ones).
