What does Arora mean by 'computational history'?
The computational history consists of the contents of the tape of the Turing machine after each step. These are the variables in the SAT instance produced by the Cook-Levin theorem, which is the combinatorial problem which represents the computational history.

This representation is fragile in the sense that even when the SAT instance is unsatisfiable, we may be able to satisfy almost all of its clauses - say, all but a constant number. This is because the reduction in the Cook-Levin theorem is very local: if we flip a single bit in the computational history, then this falsifies only a small number of clauses.
In contrast, the PCP theorem guarantees that the SAT instance is either satisfiable or at most 99% of its clauses can be satisfied, and this implies a hardness of approximation result for MAX-SAT.