Where did the saying 'Clear as a Bell' come from, what does it mean?

It means 'clearly understood'
'You don't have to repeat yourself. Your message is clear as a bell.'

Origin: Bells such as the type used in churches are large and loud. Their sound can be heard from a great distance. Bells sound a single, clear note so their sound is distinctive and not easily confused.
Before electric sirens and amplification systems, bells were a valuable means of signaling people and alerting of important events - like an impending attack. The bell and the message intended could be heard clearly over a large area.

Back in the 1910's, many companies were trying to get into the manufacturing and selling one the hottest items around, the phonograph.
One of those companies was the Sonora Chime Company.
This company started the Sonora Phonograph Company and used 'Clear as a Bell' as their slogan, touting the fidelity of their machine's sound reproduction.

Sometimes the phrase is 'As SOUND as a bell'

Shakespeare used this in 'Much Ado About Nothing' (III,ii)

'He hath a heart as sound as a bell, and his tongue is the {clapper;} for what his heart thinks, his tongue speaks.'