What is the use of retainAll(ele) in Salesforce?

Hello Prachi,
retainAll(): Retains only the elements in this set that are contained in the specified list.
Syntax:

Set<integer> set1 = new Set<integer>{1, 2, 3};
List<integer> list1 = new List<integer>{1, 2, 4, 3};
Boolean result = set1.retainAll(list1);
System.assertEquals(true, result);

Hope this will help.
Thanks

Hi,
retainAll(): Retains only the elements in this set that are contained in the specified list.
Syntax:

 Set<integer> mySet = new Set<integer>{1, 2, 3};
 List<integer> myList = new List<integer>{1, 2, 4, 3};
 Boolean result = mySet.retainAll(myList);
 System.assertEquals(true, result);

Hope this helps.

In simple words, it compares a set with another set/list and check for common elements. if there is any common element between them, it keeps the element and removes the uncommon element.