A Scots idiom encapsulating a Scottish kind of egalitarianism, central to the Scottish national identity.
(We're a') Jock Tamson's bairns = we're all the same.
'Jock Tamson' (John Thomson) could've been a historical figure - some sources claim the phrase comes from a John Thomson who was a highly-regarded Reverend of Duddingston Kirk in Edinburgh from 1805-1840. Sir Walter Scott was an elder of this church, which may have aided the dissemination of this phrase.
Alternatively, Jock Tamson could be a Scots form of 'John Thomas' which is slang for penis. We're a' Jock Tamson's bairns could mean that we're all the product of sexual intercourse - i.e. we're all created the same way.