Below is an example of how you can make add and explanation to the category selection using an action

function fp_my_custom_tax_output($thispost, $tmp_task_new, $tmp_tax_name)
   {
   if ($tmp_tax_name == "category")
       Echo '<div id="fp_tax_explanation">*** Your explanation text***</div>';
   }

// add the action before taxonomy fields	
add_action( 'frontier_post_form_standard_tax', 'fp_my_custom_tax_output', 10, 3 );

Insert the above code in your child themes functions.php

You can also add a text above the text editor, then you use this action instead:

// add the action before the text editor
add_action( 'frontier_post_form_standard_top', 'fp_my_custom_tax_output', 10, 3 );

Explanation on functions.php: https://www.wpbeginner.com/glossary/functions-php/

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