List.FindIndex

How works FindIndex?

 public class FRef : MonoBehaviour {
     string player_n;
     public List<Prop> p = new List<Prop>();
     int idx;
 
     void Start()
     {
         idx = p.FindIndex(IsAlive);
         Debug.Log(p[idx].name);
     }
 
     bool IsAlive(Prop stats)
     {
         return stats.health == 100;
     }
 }
 

And class.cs:

 using UnityEngine;
 using System.Collections;
 using System;
 
 public class Classes {
 
 }
 [Serializable]
 public class Prop
 {
     public string name;
     public int health;
 }
 

I understood what this code does but I was wondering about "FindIndex". It is very strange. Please help.