What is it?
The legal minimum hourly (or equivalent rate) you can pay someone working in the UK irrespective of the type of work being done.
One of its great flaws is that it is 'one size fits all'. For instance at the moment it is said that we can't get enough Care Workers due to poor wages for a difficult job. The fix is eeeeasy - have a supplement for those working in this industry. E.g. +£1.50/hr.
The rate paid gets beaten down by competition over time, and the NMW may not be popular with some employers, but at least they are all in the same boat. The downside for us as consumers is that Care for our mums/dads will cost more in future - nothing is free.
See link for the government guidance on this topic.