There are two correct answers:
1. A sequence of DNA that inhibits the action of enhancers in a position-dependent manner. Those are enhancer-blocking insulators and when an insulator is located in between a promoter and an enhancer, it prevents the influence of the enhancer on the promoter.
2. A segment of DNA that forms supercoiled heterochromatin that is devoid of any DNA-regulator binding. Those are barrier insulators and they prevent euchromatin (transcriptionally active region) from turning into heterochromatin (transcriptionally inactive).