A ray is a vector with a starting point and a direction (a line in 3D space). Tracing means: "where does this vector intersect an object". So there is nothing

A ray is a vector with a starting point and a direction (a line in 3D space). Tracing means: "where does this vector intersect an object". So there is nothing special about ray tracing. But GPUs now can calculate those vectors very fast. And by tracing rays we can simulate how real lights behaves. So today we can have photorealistic images in seconds or even realtime.