Literally it refers to the fact that bells are loud and have a sound with a relative lack of overtones, giving a “clear” sound.

By simple analogy, it can refer to a sound that is similarly easily heardand easily discerned.

Moreover, any signal is invisible and easily discernable. Can this be done on anything that is clearly visible or not). If it was clear as a bell, it would both be guaranteed to be excellent and sound is accurate (consider picture quality).

In common figurative use, it can refer to anything easy discerned and understood, such as instructions that were in plain language and easily comprehended.