The user tries to do something, but they are persistently interrupted by requests to do something else that may not be in their best interests.
Nagging is a form adversarial resource depletion. Every time an app or a website interrupts the user with a request to do something, this depletes the user's time and attention. This is like a tax that the provider imposes on users who do not want to comply with the provider's wishes. Although the cost is non-financial, it adds up and eventually becomes non-trivial. At this point the user may decide that it’s more cost effective to just give in and agree to whatever the provider is asking for, even if it is against their best interests.
In 2018, Instagram aggressively nagged users to turn on notifications, repeating this regularly over a period of months. Users were not able to reject the request entirely – their only option to reject was "Not Now", which allowed the nagging to continue. (Reported by Gray et al., 2018. Image source: guidingtech.com)