When people hear machine learning, they usually go to the apocalyptic "A.I. will take over the world" scenario. Fortunately (or unfortunately if you're so inclined), we're not close enough to developing AI advanced enough to become sentient just yet.
For now, Machine Learning focuses primarily on taking data and giving it to a computer to make predictions about that data. This can be in the form of predicting how something will behave, like business sales. A more common use for Machine Learning is to classify things. This is handy for astronomers, as we have a surplus of data and not enough people to look at all of it. Machine Learning can take all of this data and classify it for us, so we don't have to waste our time sifting through lots of useless stuff!