Classification is a machine learning algorithm. It classifies. Is this a caterpillar or is this a butterfly. It this a formal or casual mail. Also used in image recognition : a ball or a mug or is it a human or not or it multiple objects
First the program is exposed to samples labelled a and labelled b. Next time a sample is put in front of it it can say of which type it is
that pink line is an imaginary line helping the algo deciding of what category next samples are.