This is the term frequently applied by common law jurisdictions (England being one) to the rules applicable in many civil law jurisdictions (e.g. typically countries on mainland Europe) under which a testator is not free to dispose of his estate in any manner he may choose (i.e. basically, the law lays down certain restrictions on the distribution of his estate on death).
Technically, the jurisdictions require that a ‘legal reserve’ or ‘portio legitima’ or ‘reserved share’ apply to a proportion of the deceased’s estate. Normally, this legal reserve is exercised in favour of children, but also sometimes in favour of surviving spouses.