I was listening to Dire Straits and wondered what "brothers in arms" are...
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)
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.
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).
No answer to this question can possibly be complete without a reference to Shakespeare's St. Crispins's Day speech by Henry V (my emphasis):
We few, we happy few, we band of brothers;
For he to-day that sheds his blood with me
Shall be my brother; be he ne’er so vile,
This day shall gentle his condition;
And gentlemen in England now-a-bed
Shall think themselves accurs’d they were not here,
And hold their manhoods cheap whiles any speaks
That fought with us upon Saint Crispin’s day.