User onboarding in the context of mobile application is the journey / process of getting the user to successfully start using the app, or product powered by the app. This might include the installation, registration, activation and taking the first actions that will determine whether a user has adopted the app and/or product. Given that a lot of effort and money will be going into the marketing of your app, you need to make sure that your developers will take all steps necessary to make sure they don't "lose" a potential user during the onboarding process due to a bad user experience or any technical issues. Besides the negative experience, there is the loss of LTV revenue of that potential customer. Your developer needs to have an understanding of such implications and be able to demonstrate an understanding oh what issues might arise and what steps can be taken to mitigate them.