A ray can be thought of as a part of a line. It has a definite beginning, or endpoint. (A line does not have endpoints.) A ray continues forever in one direction only. (A line continues forever in two directions.)
The above ray has an endpoint at Point A.
Point B is also on the ray.
This ray is pronounced 'Ray AB'. It is not pronounced 'Ray BA'. The first letter in the ray's name must be the endpoint.
Imagine the ray to be as thin as possible.