That should work fine, and the alternate if(): endif; structure is a bit cleaner than the echo code which will do the same thing (even though my example had it in a mixed-up order).
Analyzing the code structure line by line:
<?php if( get_field( "review_summary" ) ): ?>
We check with PHP to see if the custom field review_summary returns any results at all. If the field is not populated, the next few lines of code gets skipped entirely. Nothing is printed or echoed here.
<h3><?php _e('Summary'); ?></h3>
Prints the title “Summary” in an <h3 /> tag. This only happens if the previous if() statement validates.
<p><?php the_field( "review_summary" ); ?></p>
Prints the review_summary field. Unlike the initial get_field("review_summary") field call, this one actively outputs its contents without having to set an echo in front of it.
<?php endif; ?>
Closing if statement.
Sometimes it helps to indent code to get a better view of the flow:
<?php if( get_field( "review_summary" ) ): ?>
<h3><?php _e('Summary'); ?></h3>
<p><?php the_field( "review_summary" ); ?></p>
<?php endif ?>
Forgive me if this is too obvious – just trying to help answer any questions you might have. Good luck!
