The second digit indicates the degree of protection of the equipment inside the enclosure against the harmful entry of various forms of moisture; such as dripping, spraying, submersion, etc. For example, a product with an IP55 rating provides some protection from penetration by dust and water projected from a nozzle. An enclosure with an IP55 rating would  not be expected to seal out all water if it were completely immersed. A rating of IP67 would be dust tight and would remain completely sealed despite being immersed in water for 30 minutes at a depth of up to 1m.