The instability strip refers to the region of the H-R diagram where stars are unstable, i.e., some of the types of variable stars, including Cepheid variables. It is a diagonal strip leading away from the main-sequence line, between giant stars and supergiants.
The basic instability mechanism is a helium layer at a temperature such that singly and doubly ionized states exist, the latter deeper, at the higher temperature. The star's expansion allows recombination from doubly to singly ionized, which reduces opacity, allowing the star to cool and contract. Contraction promotes ionization, increasing opacity and thus temperature, triggering expansion again.