Uniparental disomy means that an individual has inherited both copies (alleles) of a chromosome or parts of it from only one parent. This is called isodisomy if it is the same parental chromosome in duplicated form.
Heterodisomy refers to an allele constellation in which the two different copies of a chromosome come from only one parent.
A pathological constellation occurs when there is a mutation in a recessive gene on the chromosome that is inherited as an isodisomy from only one parent. This constellation then leads to the disease in the child by duplication of the mutated chromosome or gene.
For most chromosomes, this situation probably has no pathological significance.