Quick answer
Dielectric strength is the maximum electric field a cable's insulation can withstand before it breaks down and lets current leak through. Higher dielectric strength means the insulation is a better, safer barrier — it can handle higher voltages and surges without failing.
Along with high insulation resistance, it is a core measure of insulation quality. Premium insulation such as XLPE and the e-beam cross-linked compound in APAR Anushakti Fire Protekt offers excellent dielectric strength for reliable, long-life performance.