A choice field allows you to store a (text) value on a card which is preselected from a number of choices.
You can determine exactly which choices are selectable and in which order to present them. When editing a card a choice field is presented as a dropdown.
Choice fields can be used to group a set of cards on. This is very useful when grouping for example on the status of a card. (I.e a Task card has a choice field "status" with choices "New", "In progress", "Completed")