When to use a sandbox versus production
Key Takeaway
Build and test app changes in a sandbox, then deploy them to production. Keep the work you do in production to routine administrative tasks.
A sandbox is a copy of your Salesforce production org in a separate environment. You use it for development, testing, and training. A sandbox is completely isolated from production, so operations you perform in it don't affect production. You can safely build and test changes there before you deploy them. Production is the live org your users and customers depend on, so keep the work you do there to routine administrative tasks.
Why building in production is risky
Building or modifying apps directly in production is generally not a good idea. Even a small, untested change can have cascading effects that break a critical process, damage data, or make the system unusable for your team or your customers.
For example, changing a field's type can look like a minor change, but it can irreversibly alter data or cause Apex code to recompile. Make and test changes like this in a sandbox before they reach production.