What's the meaning of CS0659?

'class' overrides Object.Equals(object o) but does not override Object.GetHashCode()

'class' overrides Object.Equals(object o) but does not override Object.GetHashCode()

I got this warning message but i cannot find what's the reason I should override GetHashCode. Also, how should I implement it?

What's the meaning of CS0659? - C#