In GraphQL, nullability is a built-in concept that allows developers to specify whether a field can return null values. By default, every field in GraphQL is nullable, but developers can opt-in to make fields non-null, which guarantees that the field will always have a value. Non-null fields provide a guarantee for the shape of the result and ensure that frontend code never has to check for null when accessing certain fields. However, using non-null fields can be a breaking change, especially if added to existing APIs, as it can cause errors in client-side code that relies on nullable fields. To mitigate this risk, developers should carefully consider backwards compatibility when adding or removing non-null fields from their schema. Additionally, static type checking and code generation can help ensure that clients are not affected by changes to nullability. Overall, understanding nullability is crucial for building robust and maintainable GraphQL APIs.