The purpose of machine learning is to find a function through data that can map inputs to outputs. For example, given an image, this function can map the input image to corresponding descriptive text.
How do we find this function? The process of machine learning first involves setting the form of the model and the parameters to be used. Generally, models mostly use neural networks, and the parameters within are referred to as weights. A large model may have many parameters, some reaching billions. To find good parameters is beyond human capability, so the task is left to machines to compute. This computing process is known as training the model.