I matches the values in the field to the terms of the post.
Here’s an example using posts.
When you edit a post you can select 1 or more categories on the right side of the editor.
When you create a taxonomy field for categories that is shown on posts as well.
If you have “Load Terms” set to yes then whatever is selected in the standard category box will be loaded into the taxonomy field and it will override whatever is selected there.
If you have “Save Terms” set to yes then whatever you select in the taxonomy field will override what is in the standard category field.
This is useful for making it possible to only select a single category. By default WP allows multiple categories. Using WP filters you can hide the standard category box and add a taxonomy field that only allows 1 selection.
