IP ratings always have 2 numbers as there are different “levels” of water resistance that are standard.
The first digit (0-6) shows the level of protection from solids such as dust, debris and other discarded fine material.
The second digit (0-8) shows the level of protection from moisture. Moisture here is defined as any liquid contact from water sprays, drips and total submersion.
The X acts as a placeholder since there is not a particle (or dust rating). If something has a dust rating of 6 (dustproof) and a water rating of 5, you write it as IP65. In the rare instance that the dust and water rating are the same (let’s say 6), you write it as IP6.
To put it into context, the higher the number, the better the protection.
The IP rating chart below shows it clearly;