What is the Far Sync Oracle Data Guard feature?

Far Sync: 

Far Sync feature was  first introduced in 12c Release 1 and it requires Oracle Active Data Guard (ADG) License to use for production purpose.

An Oracle Standby Far SYNC instance is a  remote log transport proxy Oracle standby instance that accepts redo changes from the primary DB and write them into Standby Redo Logs (SRLs) at Far Sync instance's  local destination and then ships that redo changes to the target standby DB. It also archives those Standby Redo Logs(SRLs) to local destination at Far SYNC instance.

A far SYNC instance requires only control file and SRLs and it does not have data files and hence you can't open far sync instance to read/write data.

 Far Sync feature will require an Oracle Active Data Guard (ADG) license to use for production purpose.

It's recommended that Far Sync instance to be placed near(1-150 miles apart) to the primary DB so that there is low network latency between primary and Far Sync instance, which will help in minimizing impact on commit response time and guarantees higher data protection.

References:

https://docs.oracle.com/database/121/SBYDB/create_fs.htm#SBYDB5416
https://oracledbwr.com/step-by-step-configure-oracle-12c-data-guard-far-sync-standby-database/