An itchy skin rash caused when the body is trying to sweat but the sweat glands are blocked (with dead skin cells, oil deposits etc), perhaps from lack of use over winter. The rash is fine and bumpy - the miliaria part of the name. "Rubra" describes when the bumps more red and inflamed, responding often to and increase in bacteria or some other irritation; "crystallina" describes clear, sweat filled bumps with less inflammation.
Miliaria is commonest in hot, humid conditions but may occur in desert regions. It affects up to 30% of people exposed to these climatic conditions. It may begin within a few days of arrival in a tropical climate but is maximal after 2-5 months. There is a striking variation in individual susceptibility. Infants are especially prone.