Generally, it means that the beneficiary under the trust only has limited rights to the property. Eg, if a house they only have the right to live in it or receive the rental income; if investments they only have the right to income. However, trusts do vary and the possibilities are endless.
As to whether there is IHT, it really depends on the type of trust and the value.
If A dies leaving a house to B for his life and on his death to C. On B's death, the IHT will be calculated on the value of the house + the value of B's own estate and apportioned between them.
If you provide more information, someone will know the answer.