XSS stands for Cross-site scripting. It is a web security flaw that allows an attacker to manipulate how users interact with a susceptible application. It allows an attacker to get around the same-origin policy, which is meant to KEEP websites separate from one another. Cross-site scripting flaws allow an attacker to impersonate a victim user and execute any actions that the user is capable of, as well as access any of the user's data. If the victim user has privileged access to the application, the attacker may be able to take complete CONTROL of the app's functionality and data.

Preventing cross-site scripting can be simple in some circumstances, but it can be much more difficult in others, depending on the application's sophistication and how it handles user-controllable data. In general, preventing XSS vulnerabilities will almost certainly need a mix of the following measures:
On arrival, filter the input. Filter user input as precisely as feasible at the point when it is received, based on what is EXPECTED or valid input.
On the output, encode the data. Encode user-controllable data in HTTP responses at the point where it is output to avoid it being perceived as active content. Depending on the output context, a combination of HTML, URL, JavaScript, and CSS encoding may be required.
Use headers that are relevant for the response. You can use the Content-Type and X-Content-Type-Options headers to ensure that BROWSERS read HTTP responses in the way you intend, preventing XSS in HTTP responses that aren't INTENDED to contain any HTML or JavaScript.
Policy for Content Security. You can utilize Content Security Policy (CSP) as a last line of defense to mitigate the severity of any remaining XSS issues.