A bit of googling found these excellent explanatory notes:
The application of this common law rule, which still applies without statutory modifications to dispositions made before 16 July 1964, can be demonstrated by an example. X makes a gift of property in a will to the first of A’s children to attain the age of 21. On X’s death the will takes effect. It purports to create a property interest for the first of A’s children to meet the condition specified. The perpetuity period will begin to run on the date of X’s death and will continue for the remainder of A’s life plus 21 years. If one looks at the matter as at the date of X’s death, it is certain that any child of A will attain 21 (if at all) within 21 years of A’s death, since A cannot produce more children once dead. The gift therefore does not infringe the rule against perpetuities.
On the other hand, a gift in X’s will to the first of A’s children to become a doctor would be void at common law, assuming none of them is already a doctor. It is not certain, at the date of X’s death, that any child of A will become a doctor (if at all) within 21 years of A’s death. It is certainly possible that a child of A may become a doctor within that time. However, looked at from the date of X’s death, it is possible that the first child of A to become a doctor may not do so until after the perpetuity period has expired. Hence the gift would be void at common law.
The rule against perpetuities was originally developed in the context of family settlements to curtail control by one generation of the use of property by future generations. However, the rule was later extended to other types of property rights such as future easements, options to purchase and some rights of pre-emption
Back to your original question: this does not prevent A leaving property in their will to their child B, who leaves it to their child C in B's will, who then leaves it to D etc. It does prevent A from trying to construct setups in wills which control the ability of their inheritors to use the estate, especially setups in which the estate would continue to exist as an entity controlled (in the legal sense) only by the dead person.
(Note that in practice the combination of inheritance tax, soaring maintenance costs, war requisitions, the overtaking of feudal agriculture as a source of wealth, and the poor fire safety of pre-20th century buildings means that very few of the old big country houses are still in the private hands of the families who owned them.)
