The term is used to describe detention in prison for an indefinite length of time;[3] a judge may rule that a person be “detained at His Majesty’s pleasure” for serious offences or based on a successful insanity defence.[4] This is sometimes used where there is a great risk of re-offending. However, it is most often used for juvenile offenders, usually as a substitute for life sentencing (which might be much longer for youthful offenders). For example, section 259 of the Sentencing Act 2020 (which applies to England and Wales) states, “where […] a person convicted of murder, or any other offence the sentence for which is fixed by law as life imprisonment, and the person appears to the court to have been aged under 18 at the time the offence was committed. The court must sentence the offender to be detained during Her Majesty’s pleasure.”[5]

Prisoners held at His Majesty’s pleasure are periodically reviewed to determine whether their sentence can be deemed complete; although this power traditionally rested with the monarch, such reviews are now made in the name of the monarch, on the advice of government officials — the Secretary of State for Justice in England and Wales, for instance. Minimum terms are also set, before which the prisoner cannot be released; in England and Wales, these were originally set by the Home Secretary, but, since 30 November 2000, have been set by the trial judge.[6] Prisoners’ sentences are typically deemed to be complete when the reviewing body is “satisfied that there has been a significant change in the offender’s attitude and behaviour”.[6]

https://en.wikipedia.org/wiki/At_His_Majesty's_pleasure#In_penal_law

It’s specifically not life in prison because minors aren’t fully responsible for themselves and life from the age of 10 and life from the age of, for example, 55 are completely different things.