neural networks – Cant understand that is an activation function

Okay i have been trying to learn neural networks from scratch , in python , so first ,wherever they talk about neural networks , they never mention what do you mean by activation of a neuron , i dont get this and need to understand the basic meaning of activation , is it like activated – non activated (come kind of boolean) and if we see the definition of bias it is that it helps to compensate the activation of the neuron. and y do we need an activation function at all , what is it doing im not able to understand