Over a period of many years, a large number of contractors working through their own limited companies signed up to a series of tax schemes that appeared too good to be true. Essentially, the idea was that, rather than paying yourself an income, your company paid the money to a trust instead, which then just lent the money to you. Since you now had a loan, but no income, you had no income tax to pay. Brilliant!
The majority of contractors didn't sign up to such arrangements, because they thought they probably actually were too good to be true. They were quite right of course. These schemes have all failed, and the high earners who tried to avoid paying any tax at all by pretending that they were just borrowing money on a temporary basis are going to have to cough up, quite rightly. 5 April 2019 is a significant date - if the recipient hasn't repaid the "loan" (that wasn't really a loan) by then, they'll have to pay all the tax and NI that would have been due on that amount of net income. If they do repay it, they'll subsequently have to then pay themselves properly and pay the appropriate tax.
There's been a lot of disquiet from the people caught out over this, who thought that paying no tax at all on high income was a reasonable course of action to take, but, if it was a loan, then logically it shouldn't be a problem - they always knew that they'd have to pay it back, right? That's what loans are. Surely they wouldn't have spent it all on holidays and cars. If you've not made arrangements to one day repay it, then you can't really have considered it a loan at all - it's almost as if you knew it was permanent income all along...