ReLu definition :

The Relu is an activation function defined as follows  :

f( x ) = max( 0 , x )

which simply returns its argument x whenever its greater than zero and 0 otherwise.

Also, one of the major advantage of the ReLU activation function  is to allow the back-propagation of the error signal while still maintaining a non-linearity characteristics which enables it to behave analogously to a biological neuron and therefore model complicated non-linear functions.