Keli Sheni

Question

With regard to stovetops and counters you have written that since they are a keli sheni there is no problem with the hametz that might have fallen on them. Can you explain in more detail the concept of keli sheni. I gather from what you have written that hametz and other foods can't come out of a keli sheni, which also seems to mean that from a kashrut standpoint there is no need to ever put down a tablecloth.