The ingress protection rating (i.e. IP rating) refers to the internal protection of the device from external elements – namely liquids and small particles (e.g.dust).
The first number refers to the level of the protection of the device from small solid particles. In the case of the Nothing Phone (1) that number is 5, which translates to “dust protected”. This means that ingress of dust is not entirely impossible, but it is very difficult for a sufficient amount of it to enter into the device and interfere with its internal functioning.
The second number of the rating refers to the level of protection from liquids. This is the part of the index that determines whether a device is waterproof per se. The rating of 3 indicates that Nothing Phone (1) can survive being sprayed with water over a period of time. However, in order for a device to be able to sustain full immersion, it must have an index of 7 or above.