No insulation is perfect: it always lets a small current through to earth or to another conductor. That leakage current is the direct counterpart of insulation resistance, and it grows as the insulation degrades, absorbs moisture or accumulates conductive dirt on its surface.
It has two readings. As a condition indicator, its trend over time is an early sign of deterioration. As a safety matter, it is the current a residual-current device detects and the one that could flow through a person touching the enclosure, hence the regulatory limits in installations and in medical equipment.
In a DC insulation test the total current has three parts: the capacitive component, which vanishes in seconds; the absorption current, which decays over minutes; and the leakage current proper, which is what remains at the end and what needs watching.