Learn Swedish: De kom aldrig överens.

It is part of the phrasal verb komma överens. You can think of it as literally "coming to an agreement." This is the standard Swedish way to say "to agree" on something, or "to get along" with someone.

Why is aldrig placed between kom and överens?

In a standard Swedish main clause, a sentence adverb like aldrig (never) or inte (not) usually goes immediately after the verb. Because komma överens is a two-part verb, it splits up, and the adverb steps right into the middle.

Can I use var (was/were) instead of kom here?

Yes, but it changes the meaning slightly. De var aldrig överens means they were never in a state of agreement. De kom aldrig överens focuses on the action, meaning they never reached an agreement, or simply that they never got along as people.

What does överens actually mean on its own?

It translates roughly to "in agreement" or "at one." However, you will rarely see it floating around on its own; it is almost exclusively glued to either komma (to reach an agreement/get along) or vara (to be in agreement).