What does the expression "Brothers in Arms" mean?

I was listening to Dire Straits and wondered what "brothers in arms" are...

Brothers in arms are soldiers fighting on the same side of some conflict.

Here, brothers is not used literally but to mean "ally" or "comrade". The in arms part refers to the fact that they are soldiers, arms meaning "weaponry".

It's referring to fellow combatants or soldiers.

Through these fields of destruction
Baptisms of fire
I've witnessed your suffering
As the battles raged higher
And though they hurt me so bad
In the fear and alarm
You did not desert me
My brothers in arms

It is ostensibly a song about camaraderie in war, but it could be metaphorical (as is so often the case with songs and poetry)

It means fellow soldiers.

Maybe not always in a military sense, but since soldiers have to rely on their colleagues, it can mean someone you trust in any situation where there is opposition.

Brothers-in-arms simply means comrades or comrades-in-arms. -In-arms is synonymous with in battle, at war, etc... (literally or figuratively). By and large, the term evokes solidarity, especially in a struggle.

comrade (noun)^NOAD

a companion who shares one's activities or is a fellow member of an organization.

• (also comrade-in-arms)a fellow soldier or serviceman.
• a fellow socialist or communist (often as a form of address).