“A dime a dozen” is a phrase which is used in reference to anything which is common and/or cheap. A dime is an American unit of money equal to ten cents (a small amount of money), and a dozen means twelve of something. In other words, the thing is very cheap, less than a penny each.
“Experts in this field are a dime a dozen.” (It means experts in a certain industry were very common and/or didn’t make much money.)
“Smiles were a dime a dozen at the Thanksgiving Day Parade.” (It means smiles were very common.)