What does ‘legally ringfenced’ mean?

It has been a while since my last post but today’s announcement of the increase to national insurance (by 1.5%) got me thinking, what does “legally ringfenced” mean?

I refer, of course, to the government rhetoric that this increase will be in some way ringfenced for the NHS in a permanent manner. Mention of this can be seen at this tweet by The Mail’s Political Editor: https://twitter.com/JasonGroves1/status/1434995370086502401?ref_src=twsrc%5Etfw

This surely raises significant issue with the concept of Parliamentary supremacy. Nothing in the UK can ever be prevented from change forever. Parliament is always able to make new Acts that unmake previous ones. This is a fundamental constitutional principle of the UK.

I defer to the ever brilliant and succinct David Allen Green for his excellent summary of this point here: https://davidallengreen.com/2021/09/why-legally-ringfenced-is-a-phrase-used-by-political-knaves-to-take-you-for-a-fool/) but needless to say, political priorities change and do not be surprised if the ‘ringfence’ is kicked down in future years.