That's simple. It's a "rash" on the skin that is usually caused by friction against a rough surface or heat such as working outside and sweating and rubbing off sweat could cause a heat rash on your face, or you might get one under your arms or in the bend of your arm.
Heat rash is a rash that occurs when sweat is trapped under your skin. Some of the symptoms include raised bumps on your skin in the area that is affected. For treatment you can try some topical creams or see a doctor.