"On yer plates, lads!" might sound (almost) like a shout from a baseball coach to the ears of people on one side of the Atlantic. However, in parts of London it would have a quite different meaning: "Stand up, boys!"
Cockney rhyming slang takes an expression that rhymes with a word and then uses the expression instead of the word.
Thus, 'feet' become PLATES OF MEAT.
Often the whole phrase is used but sometimes the rhyming word is dropped.
See how many of these you know - or make an educated guess! Remember, the rhyme is at the end of the slang expression.