Marshalling (and unmarshalling) is about converting data in one format to another format. For example, converting a [[json]] object into a Go struct (and vice versa).
In the case of GraphQL/gqlgen, unmarshalling converts it into a standard GraphQL type, where as marshalling converts it into a custom type (e.g. an enum).
This is very similar to serilization, depending on the language you are working with