Secrets Trusts are often used to look after children born outside marriage. Or for a secret lover, or perhaps just someone your family disapproves of.
The principle is that the person leaves certain assets or sums to a particular person who appears to be the beneficiary. In fact, that person has promised to pass the inheritance on under the secret trust to a third party who has not been named in the Will. Consequently, the gift is secret, even after the Will becomes available for public viewing after probate has been granted.
Here is a really good video by City College, if you would like to learn more.