Bare root plants are plants in a dormant state that do not have soil around the roots and are thus 'bare'. Although it may not be how you are used to planting, it is the best way to plant if you have the option, primarily for benefits to the long-term root structure of the tree as well as for increasing the chances of establishment.
Before the advent of plastic containers, bare root was the default state in which plant material was moved from place to place. It is still the the default way that trees are shipped all over the world from web and catalogue sales.
Bare root trees grow quicker and adapt better to their planting site. They have greater root mass than potted trees, and overwhelmingly they have better root structure which leads to greater long term growth and survival.