Monolithic applications have low reliability when it comes to system uptime, as one small issue in a particular module can bring down the entire system. Dividing your application into multiple small services reduces the impact area, so that issue is one part of the application shouldn't impact the whole system, and the application can continue to serve critical functionality. For example, in an e-commerce website, an issue with the payment service should not affect the customer's ability to place orders, as payment can be processed later.
At the service level, scale your application horizontally to increase system availability. Design a system so that it can use multiple smaller components working together rather than a single monolithic system to reduce the impact area. In a distributed design, requests are handled by different components of the system, and the failure of one component doesn't impact the functioning of other parts of the system...