An IP55 rating on a night vision rifle scope means that the scope is protected against dust and against water jets.
IP stands for "Ingress Protection" and it is a standard to measure the protection against solids and liquids. The first digit (5) in IP55 indicates the scope's level of protection against solids (like dust) and the second digit (5) indicates the scope's level of protection against liquids (like water).
IP55 rating means that the scope is protected against dust ingress and it is also protected against low-pressure water jets from all directions. This means that the scope can withstand water splashes and short-term exposure to water, but it is not meant to be submerged in water or exposed to high-pressure water jets.
It is important to note that the IP rating only applies to the scope and not the whole rifle as well as it is not a permanent protection and can be damaged if exposed for a long period of time or if the scope is not properly closed or sealed.