for the error policy, you basically control the behavior (for each client instance or each request) when a request is considered failed. none, ignore and all. [1]
fetch policy allows you immense control over caching. this all depends on what you're doing but in some instances it can even make sense to never cache any requests, depending on how you application is structured. the docs are hard to google for this, but here's a link for you [2]
[1] https://www.apollographql.com/docs/react/features/error-hand...
[2] https://www.apollographql.com/docs/react/api/react-apollo.ht...