It is the Celtic equivalent of a 'soul mate'. A friendship that cuts across gender, culture, race and all man made divides. The connection between souls that is healing, energizing, empowering and enabling. when you have a 'Anam Cara' the bond between the two will be resilient, robust an rewarding.
There is a spiritual connotation to this link. There is a hint of an ancient and everlasting connection too.