A cold, well-ventilated attic is a healthy attic. Ventilation carries away moisture that would otherwise condense on the underside of the roof, and in winter it keeps the roof deck cold so snow doesn't melt and refreeze into ice dams at the eaves.
How much ventilation
A common code guideline is a minimum of 1 square foot of net free vent area for every 300 square feet of insulated attic floor, split between intake (soffit) and exhaust (ridge or roof) vents. Balanced intake and exhaust is what makes air actually move.
Keep soffits clear
Insulation that's stuffed into the eaves blocks soffit intake and defeats the system. Baffles hold insulation back from the soffit so air can flow up from the eaves and out the ridge.
Signs of trouble
Frost or moisture on the underside of the roof sheathing, mould, matted insulation, and recurring ice dams all point to poor attic ventilation or air leakage from the house below. Fixing air leaks and restoring airflow usually solves it.