What does the word 'knocked' mean in the old song "Knocked 'em in the Old Kent Road'?

to knock 'em out or knock 'em dead are idiomatic phrases, using knock as in, to hit them in the head, to concuss.

knock them/'em dead (informal) to perform so well, or to look so attractive that other people admire you a lot

You'll knock them dead at the party tonight in your new black dress!
Just go out there tonight and knock 'em dead! (often an order)

In this case, I'd say they were saying Bill knocked 'em dead, as in a wonderful performance.

Susan has answered the question, but I found this bit of information connected to the song, which I thought was interesting and worthy of attention.

Phrase Finder says of this song:

The song, 'Knocked 'em in the Old Kent Road' written and performed in 1882 is also the source of the pronunciation of 'wotcher' as an exclamation - 'watch yer', as opposed to the original query - 'what cheer?

"Wot cher!" all the neighbours cried,
"Who’re yer goin’ to meet, Bill?
Have yer bought the street, Bill?"
Laugh! I thought I should ’ave died,
Knock’d ’em in the Old Kent Road!

In the verse, Bill laughed so much he almost died. Original song here performed by the author himself, Albert Chevalier. The son of a Frenchman and a Welsh mother, his full name was Albert Onesime Britannicus Gwathveoyd Louis Chevalier.

Knocked 'em dead
used to tell someone to perform or play as well as they can

"Knocked" = "stunned"

And someone came up with the interpretation "watch yer" for "wotcher"; actually it's the old greeting "What cheer?", meaning, "what news" or "what's keeping you happy at the moment?" Very common in lota of parts of England, particularly London and Tyneside.