Explanation:
Adverse selection is a term associated with the situation where "a buyer knows few details which the seller is unaware" and vice versa.
The buyer do not match with the expectation of seller and vice-versa.
Best example is the "insurance policy".
The insurer who works in dangerous job like race car, working with explosives, etc but the insurance company does not know that he / she is working in high risk job.
Also sicker people are more dangerous to the insurance company, since they will claim for sure.