When fuel nuclei split apart in a nuclear reactor, their split fragments form a variety of lighter elements. Some of these elements are so-called "noble" metals -- metals that tend toward remaining chemically stable as metals, rather than as chemical compounds with other elements. Silver, ruthenium, and palladium are some examples of noble metals. More typical metallic elements like potassium and iron, by contrast, tend to be found on Earth as oxidized chemical compounds rather than as metals.

In a molten salt reactor, most fission products either stay dissolved in the salt mix or are lost from the mix as stable gases. The noble metals are different. Their tendency is to reform as solid metal. They tend to accumulate as a metallic layer or plate of metal over other solid surfaces they come into contact with. That's what is meant by plating out.

Here is a document that specifically addresses noble metal plate-out in molten salt reactors:

http://www.skyscrubber.com/Molten-Salt-Reactor%20Technology%...

See section III.C.