Segments are subsets of subscribers created based on their actions or properties. Segmenting allows you to send targeted, personalized messages and engage subscribers who’ve browsed a product on your website, added a product to their cart; additionally, you can segment by device type and device operating system. 

Segments are used when you're setting up campaigns and journeys so that you can reach the right audience. 

For further information, see:

Segmentation: How-To Guide

Video: Create segments