This is trading goods secretly in order to avoid paying customs duty (tax on imports).
Smuggling had never been a major worry but the policies of the government turned it into a major crime.
The government clamped down on smuggling in the 18th Century as it needed money to pay for wars, mostly against France.
There was no income tax, so the government had to raise money from customs duties on popular imported goods such as chocolate, tea, wine, spirits, salt, leather and soap, and excise duties on domestic goods.
Goods like tea had 70% tax on them, meaning that people were willing to buy cheaper, smuggled goods on the Black Market (illegal trade) which didn’t have tax on them.
Since there was insufficient policing to patrol the coast it was easy to smuggle goods into Britain.