The receiver drier is normally mounted in the high pressure line, between the condenser and the TX valve. It receives liquid refrigerant from the condenser, which it stores until required by the TX valve.
The receiver drier is a sealed metal canister which contains:
Filters to remove impurities, and desiccants to absorb moisture.
High pressure liquid enters the receiver drier and passes down through the filter and desiccant material. It exits via the pick-up tube at the bottom of the canister and moves on to the expansion valve.