Customary neglect is more often referred to as “salutary neglect.” This was the British policy (an unofficial policy) of leaving the colonies alone to govern themselves. The British pursued this policy for most of the colonial era before 1763.
This policy helped to bring the Revolution about because it made the colonists used to the idea that they would be in charge of their own affairs. The British did not intrude on the colonists so the colonists came to think that they had the right to be free from British intrusion. After 1763, the British government tried to exercise more control over the colonies. Since the colonists were used to salutary neglect, they became very angry when the British did this and the Revolution soon ensued.