persistent or Stored XSS is a type of XSS vulnerability and differs from reflected XSS by the fact that it is stored in nature. It means that the payload (the injected code), once inserted into a page, will remain and execute forever on the page. This is sneakier than its reflected counterpart as most of the time the victim is simply unaware of the fact that a malicious code is running on the affected website, making it an excellent option for XSS worms.
Stored XSS vulnerability is much common in places where data is saved for a longer time, for example, in places such as comment sections, messaging, and similar places. They are a welcoming location to check for stored XSS issues.