What does this mean?
Usually (at least in HMRC services) the in-page back-link uses the browser history state to take the user back to the previous page. This note indicates that the back link has been hard-coded with a url and so may behave differently.
Impact on users
Coded correctly there should be no impact for a user, in fact hard-coded urls can be better for users, but just like history-state back links there can be situations where it does not take the user where they expected and the higher technical overhead can introduce errors.
How to fix
As long as the link takes the user where they expect there is no issue with a hard-coded back link. However make sure you check situations like arriving at the page from a non-linear journey - for example from a Check Answers page, or when a user uses the browser back button and then the back link.