could you please answer this question

  
    
    
    Use of .backward() 
  
  
    what is the use of backward i cannot understand 
x=torch.tensor(3.,requires_grad=True) 
w=torch.tensor(2.,requires_grad=True) 
b=torch.tensor(4.,requires_grad=True) 
y=w*x+b 
y.backward() 
x.grad 
w.grad 
b.grad 
could you please explain use of .backward class also use of .grad