For neural networks the parameters \( \boldsymbol{\Theta} \) are given by the so-called weights and biases (to be defined below).
The weights are given by matrix elements \( w_{ij}^{(l)} \) where the superscript indicates the layer number. The biases are typically given by vector elements representing each single node of a given layer, that is \( b_j^{(l)} \).