"IP" is the normal abreviation for the IEC 60529 international standard that defines the resistance of a product to intrusion (or "ingress") by various environmental hazards. The IP-55 standard covers dust as well as low pressure water jets from any direction (often known as “driving rain” protection). Kenwood radio products carrying the IP-55 rating are therefore built to this high standard of protection against the sort of factors found in normal use.