The term "metaphorically speaking" refers to the practice of using a metaphor, or a figure of speech, to suggest a resemblance between one object and another. The sentence is not intended to be taken literally, but it is used as an implicit comparison or a representation.
An example of speaking metaphorically is when a person says, "Your proposal has no teeth." The intent is not to suggest that the proposal should literally have physical teeth, but rather that the proposed idea is not able to stand up to scrutiny or is unenforceable. In other words, the proposal has no "bite," which is yet another metaphor.