How to check for the ground in Unity 2D for Jumping

So I have this script where I use a timer to prevent jumping, but this is sort of overrided if you jump at a high block and have enough fall time to get the timer to zero, also it is weird for me if I have to wait a second or two even if I'm on the ground and needing to jump. What I want to do is have something check for the ground, and then set a variable to true if im on the ground. If that variable is true, then I can jump. Also I have heard of linecasting, but I don't understand how it works, so if that is your solution can you tell me what the code does along with how linecasting works, because I don't get the Unity description of it.

Here is the script I have right now called "Movement Test":

 #pragma strict
 
 var movementSpeed : float = 5.0f;
 var jump : float = 0;
 var jumpSpeed : float = 5;
 var jumpTimer : float = 0;
 
 function Start () {
 
 }
 
 function Update () {
 if (jump == 1){
     jumpTimer = jumpTimer + 1;    
     if (jumpTimer >= 85) {
         jumpTimer = 0;
         jump = 0;
     }
 }    
 
 
 // This is how the keys work
 if(Input.GetKey("a") || Input.GetKey("left")){
     transform.position -= Vector3.right * movementSpeed * Time.deltaTime;
     } else if(Input.GetKey("d") || Input.GetKey("right")){
         transform.position += Vector3.right * movementSpeed * Time.deltaTime;
     }
 if(Input.GetButtonDown("Jump") || Input.GetKey("w")){
         if (jump==0) {
         rigidbody2D.velocity.y = jumpSpeed;
         jump = 1;
     } 
 }
 }