Slow pointer and fast pointer are simply the names given to two pointer variables.

Slow Pointer

Travels the linked list one node at a time

Fast Pointer

Travels the linked list two nodes at a time.

Most Use Cases

1. getting middle element of linked list in one pass.
2. whether loop is present in Linked list. etc

Example

Getting middle element of linked list in one pass, lets see how:

1. Take 2 pointers, fastPointer and slowPointer and initialise both to list head.
2. Iterate through list and move slowPointer 1 node at a time and fastPointer 2 nodes at a time.
3. When fastPointer reaches end of list, slowPointer will be pointing to middle element of list.

public ListNode middleNode(ListNode head) {
        ListNode slow = head, fast = head;
        while (fast != null && fast.next != null) {
            slow = slow.next;
            fast = fast.next.next;
        }
        return slow;
    }

Reference

http://javabypatel.blogspot.com/2016/10/find-middle-element-of-linked-list.html https://leetcode.com/problems/middle-of-the-linked-list/solution/