Resolution

It builds a dependency graph starting from the entry point file.  This is required to find out what are all the files referred in the project.  For this step it uses a resolver (it's a module part of Metro). This stage actually starts in parallel with second phase of 'Transformation'

Transformation

All dependency module(files) found in the above stage will be passed to 'transformer' which is responsible to convert those into understandable format by the target platform (in this case its React Native). This convertion  process is called transpilation and it transile as many as file based on number of core available in the machine

Serialization

Once transformation step over then this module will combines all into one single or multiple JavaScript file called bundle

Note

Each stage of Metro is implemented in different modules. ie., it has a module of resolver, transformer, and serializer.  You can swap these modules based on your needs

For more details Facebook Metro