what is the different between Buffer Cache that displayed from free command VS the available memory

we have 463 RHEL 7.6 machines in the cluster most of then are HDFS machines ( datanode ) from free -g command we can that usually **buff/cach**e is around 30-50 when total memory is 256G as I know - a buffer is an area of memory used to temporarily store data while being moved from one place to another but the available memory is also memory that used for application so I am little confused , what is the diff between **buff/cache** to **available** memory ?