Does rectal bleeding signify a serious complication?

Yes. A lot depends on your age. In children it can be a malformation of the intestine called a meckel's. This is easy to treat and diagnose. The diagnosis spectrum can be superficial as in hemmorhoid to internal such as indicator for cancer. Either way tests can be run to det.
Sometimes. This can be a symptom of benign hemorrhoids or cancer. All rectal bleeding should be evaluated.