Machine learning is a type of artificial intelligence in which computer algorithms are built to learn to perform set tasks, then use the data acquired to improve the performance of the processing of that data.
At the macro-level, ML can be applied to look at health records of different segments of the population (eg. elderly, pregnant, asthmatic, stroke etc.) to assist with evidence-based policy decisions. At a lower scale, ML techniques can be applied to data acquired from an individual patient's brain, heart, muscles, medical images etc. and cross referenced against "healthy" data to aid diagnosis.
Large amounts of information are often necessary for diagnosis and treatment of a condition. The volume and complexity of the data makes the physician's job difficult and provides a compelling reason for the development of machine learning based tools to aid them.