On a guide's profile page, you can give a star rating to the guide, with 0.5 being the lowest and 5 being the highest. 0 star means no rating has been given to the guide. The rating increases by 0.5 with each click.
To remove the rating, click on the left of the rating stars.
When assigning a task, the guide list (under the tabs 'Matched guides' and 'Other guides') will be sorted from the highest to the lowest star rating.