A relative clause is a grammatical structure that is “embedded” somewhere inside a sentence. The relative clause cannot stand on its own. Instead, it is contained by another sentence constituent, usually a noun phrase. Like all clauses, a relative clause must have at least a subject and a verb and may have an object and other grammatical phrases as well.
In the following sentence the underlined relative clause has a subject (which), a verb (has), and an object (a solid state image sensor).
A camera which has a solid-state image sensor is a digital camera.
The relative clause is contained inside the main clause portion, A camera… is a digital camera. Specifically, it is contained inside the noun phrase a camera to form a larger noun phrase, a camera which has a solid-state image sensor.