Hazard and Operability study aka "what if" is a technique applied by a team on a project design, or part of it, to identify and assess hazards and problems that may arise in operation; lately done in a systematic way to reduce the chance that something is missed. Risks tend to concentrate on start-up, shut-down and abnormal or transient operating conditions. The purpose is, of course, that the consequences of those risks be prevented already in the design stage.
If you can get access to Trevor Kletz's HAZOP and HAZAN "Identifying and Assessing Process Industry Hazards" published by the Institution of Chemical Engineers (UK) you'll get a better idea with worked out examples.