While developing your applications the OuSystems platform validates your implementations and issues errors and warnings, depending on the severity of the problem.
Both errors and warnings can occur at any stage of the module life cycle like implementing some logic, designing a screen, testing a query, or publishing to the server.
In OutSystems, an error is a problem that prevents deploying the module to the server.
Only when all errors are fixed, the module can be deployed to the server.
In OutSystems, a warning is a potential problem but does not prevent the deployment of a module to the server.
However, it's advisable to check the warnings and solve them, since they may be tip of the iceberg for unexpected or bad behaviors.