DSLRs and compact system cameras sometimes collect spots of dust on the sensor. The makers get round this by applying a high-frequency shaking action to the sensor to shake it off. This happens automatically when you switch the camera on or off but you can also start it manually.
Sensor cleaning can be an automatic process carried out by the camera to shake any dust particles from the sensor, but sometimes manual (user) cleaning is needed. This requires a special sensor brush (‘dry’ cleaning) or a swab and sensor cleaning fluid (‘wet’ cleaning). Manual cleaning needs a degree of skill and confidence.