A1 is the northbound interface in the RIC for policy guidance. The northbound caller (non RT RIC, SMO, etc) creates policy types and policy instances through A1, and subsequently messages are exchanged with xApps via RMR.
Please see here for the docs: https://docs.o-ran-sc.org/projects/o-ran-sc-ric-plt-a1/en/latest/index.html
This wiki page may be useful for discussion about the A1 Mediator, feel free to post comments here.