This is the 4626th most frequent Turkish word.
birbirini
"Each other" or "one another."
In this sentence, "birbirini" indicates a mutual action between the subjects (they). It refers to the action of knowing directed from one individual to the others reciprocally.
Onlar birbirini çok iyi tanıyor.
They know each other very well.
"Birbirini" in this instance conveys reciprocal support among the subjects, expressing mutual support.
Arkadaşlar birbirini destekledi.
The friends supported each other.
Here, "birbirini" shows a reciprocal emotional relationship of love among the subjects, indicating mutual affection.