A mongos is a routing service used in the MongoDB database system. It acts as an interface between client applications and the database shards, directing queries to the appropriate shard based on the data's location. This helps in managing large datasets efficiently by distributing the load across multiple servers. In a sharded cluster, multiple mongos instances can be deployed to handle incoming requests. Each mongos instance can route queries to the correct shard, ensuring that data retrieval and updates are performed quickly. This architecture enhances performance and scalability, making it suitable for applications with high data demands.