The only time metadata components are overridden is when you have modified an existing component and you are deploying that component.
In your example, the field you created in production will be unaffected.
To illustrate the opposite effect, suppose you have a formula field you created in production. In your sandbox, you have modified the formula for that field. When you deploy that field, the formula will be overridden with whatever was in the sandbox.
This concept can get very detailed with understanding what exactly constitutes a metadata component. For example, based on the fact that you asked this question, I assume you consider the object itself to be the component and therefore deploying a validation rule tied to that object will overwrite changes made. Salesforce, however, treats each individual field, validation rule, object, list view, etc as individual metadata components.
I hope this helps you understand!