What is meant by heterothallism?
please explain in brief.

Heterothallic species have sexes that reside in different individuals. The term is applied particularly to distinguish heterothallic fungi, which require two compatible partners to produce sexual spores, from homothallic ones, which are capable of sexual reproduction from a single organism

So, in heterothallic organisms an organisms is only male or female, unlike in homothallic organisms where a single have both male and female sex organs