It means that the amount of time she can stay is purely up to the IO, even if she has the visa.
For example, if she comes in and stays for six months in one year, the next year the IO will see that she spent six months in the UK previously. This is not something a typical visitor can do. So on her next visit she can present for a six month entry with her visa but the IO will look at the number of times she has visited and decide whether they want to grant her another six months, or a shorter length of time, or no time at all.
They will use the amount of time she spends in the UK , and what visa she enters on, to determine whether she is a genuine visitor. If they feel she is not, then they can choose to grant less time, or no time at all, in the UK, to her.