The IP Code, or Ingress Protection Code is a standard which classifies and rates the degree of protection provided by mechanical casings and electrical enclosures against intrusion, dust, accidental contact, and water. It is published by the International Electrotechnical Commission. The equivalent European standard is EN 60529.
For example a typical Wiska IP66 junction box like this https://amzn.to/3x5nAzP will provide complete protection against dust and powerful waterjets or heavy seas.
The chart below shows what each number means.