Internal redirects from trailing slash mismatch
This means that the URL in question is redirected to another internal URL, where the only difference between URLs is the presence/absence of a trailing slash.
Why is this important?
A redirected URL, typically 301 (permanent) or 302 (temporary) means that the location of the page has changed, and the user is sent from the original URL to a new one. Since content gets changed and moved all the time, it is relatively common for redirects to occur, however, they make the experience worse for both users and search engine crawlers.
Redirects add an extra 'hop', which means it takes longer for the content to become available, which is a bad user signal, and means that search engine crawlers have to do extra 'work' to find the content.
For internal URLs in particular, where the website owner can control the experience, redirecting URLs should be avoided if possible.
What does the Hint check?
This Hint will trigger for any internal URL which returns a HTTP status code of 3XX, where the redirected URL adds a trailing slash to a URL without one, or removes a trailing slash to a URL with one.
Examples that trigger this Hint:
Consider the URL: https://example.com/page
The Hint would trigger for this URL if it returned a 3XX HTTP header response which adds a trailing slash, for example;
Inversely, consider the URL: https://example.com/page/
The Hint would trigger for this URL if it returned a 3XX HTTP header response which removes the trailing slash, for example;
These are both also true for any other 3XX HTTP response.
How do you resolve this issue?
To resolve this issue you first need to understand what causes it. This issue in particular is due to a server setup, where the server expecting to see URLs with a trailing slash, and so when it encounters a URL without one, it automatically redirects to a URL with one (or vice versa - the server is expecting URLs without trailing slashes, so is redirecting URLs that do not have one).
This mechanism is a catchall failsafe, designed to avoid duplicate content, however it can cause redirects when you do not use a consistent internal linking structure on the website. As with all internal redirects, the issue is not that they exist in the first place, it is that you have internal links pointing to them, instead of the correct destination URL.
The way to handle internal redirects is to identify all the links that currently point to the redirecting URL, and change the href target to instead point to the correct destination URL.
In this case, you will need to identify which URLs need manually updating, due to links with/without trailing slashes being erroneously used within content. Separately, it would be helpful to identify which URLs appear to have the erroneous link within a part of the page template (e.g. in the header). Highlighting these to your developer would be particularly helpful, as they just need to update a single file, and this would fix lots of redirect links at once.