The ReLU (Rectified Linear Unit) is the go-to function for many neural networks since it is cheap to compute and still works well enough for many applications.
It is a non-linear function that gives the same output as input if the input is above 0, otherwise the output will be 0. that is,
Output = input, if input is above 0
Output = 0, if input is below 0
The ReLU function also helps with the problem of vanishing gradients in deep networks by not squashing in both ends.