In traditional web services, a frontend application (like a webpage or an app) interacts with a backend application (like APIs or a Postgres server).
In blockchain infrastructure, a frontend speaks directly with the blockchain network, visualizing the state of the chain. A blockchain network contains nodes, which validate transactions and add them to blocks. This decentralized network of nodes takes the place of the database and APIs in a traditional web service. The blockchain itself serves as a data store, and the application (or state machine) that the nodes run take the place of its APIs (by ingesting transactions and defining transitions between states).
Note: In some cases, blockchain applications can also interact with a backend which in turn connects to the blockchain. For example, backend infrastructure can save information to an external database to serve complex queries that would be expensive to perform on the chain itself.