English 'are' is a Northumbrian dialect form that overthrew the original Old English plural 'sindon' during the Middle English period — a northern insurgent that conquered Standard English, likely aided by Old Norse influence in the Danelaw.
Second person singular and all persons plural present tense of 'be.'
From Old English 'eart' (thou art) and 'earun/aron' (are, plural), from Proto-Germanic *ar- (to be), possibly from PIE *h₁er- (to move, to set in motion) or a variant of *h₁es- (to be). The Old English plural was originally 'sindon' (from *h₁es-), but the Northumbrian dialect form 'aron' spread south during the Middle English period, eventually replacing 'sindon' entirely. The triumph of 'are' over 'sindon' was a dialectal revolution — a northern form conquering the south. Key roots: *h₁er- or *h₁es- (Proto-Indo-European: "to move, to set in motion / to be").
'Are' is actually a northern English dialect form that invaded the south. Old English originally used 'sindon' for 'they are' (from the same root as 'is'). But the Northumbrian dialect used 'aron,' and during the chaos of Middle English — with Norse influence heaviest in the north — 'aron' spread south and killed 'sindon' entirely. Modern Standard English 'are' is a dialectal insurgent that won.