Machine Learning is a subfield within Artificial Intelligence that builds algorithms, or models, that allow computers to learn to perform tasks from data instead of being explicitly programmed.
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In order for a model to learn to transform an input to the desired output, it needs to be trained with a "training set". A training set is a set of text data that will serve as examples from which the Machine Learning model will learn to perform the desired task. 

If you are more interested in this, we strongly recommend reading our Gentle Guide to Machine Learning blog post.