There are two modes of instance selection. You can select instances by clicking anywhere on the instance row, or you can select instances by setting the checkbox. Each mode of selection has a different use.
When you select an instance by clicking on the row, the instance will be highlighted and the instance will be shown in the Instance Details panel. You can only select one instance at a time with this method, since only one instance can appear at a time in the Instance Details panel.
This documentation refers to this as a highlight selection.
When you select an instance by marking the checkbox, the Operations menu is enabled. You can select many instances with this method, and any operation you perform will be done on all of them. Instances selected via checkbox do not appear in the Instance Details panel, which will always show the instance currently highlighted regardless of its checked state.
This documentation refers to this as a checkbox selection.