data-target is used by bootstrap to make your life easier. You (mostly) do not need to write a single line of Javascript to use their pre-made JavaScript components.
The data-target attribute should contain a CSS selector that points to the HTML Element that will be changed.
Modal Example Code from BS3:
In this example, the button has data-target=”#myModal”, if you click on it,
will be modified (in this case faded in).
This happens because #myModal in CSS selectors points to elements that have an id attribute with the myModal value.
Further information about the HTML5 “data-” attribute: https://developer.mozilla.org/en-US/docs/Web/Guide/HTML/Using_data_attributes
The toggle tells Bootstrap what to do and the target tells Bootstrap which element is going to open. So whenever a link like that is clicked, a modal with an id of “basicModal” will appear.