Training is the process of creating and teaching an ML model how to do something.
The way you train a model is by showing it many, many different situations and what the correct outcome is in those situations. The model uses some fancy math to learn the patterns in those situations and learns to apply them to new data. And like teaching a kid to do something, the way you train a model – from the method to the algorithms used – vary slightly depending on what you want the model to do.